My loved 17" MacBook Pro early (2009) its starts then it other times NO! New HD?

I agree with @John Galt's astonishment that any reputable tech would just replace the battery without being 100% certain the battery was the source of the problem. The tech could easily have disconnected/removed the battery to verify the laptop was functioning properly....this is possible with those older Macs, but not the USB-C Macs.

A bad HD is a real possibility, but so is a GPU failure which was really common with the 15" & 17" models of the 2008-2011 models. Bad memory or a bad memory slot is also another possibility. There is no way to know without proper testing & troubleshooting. A reputable tech should have no problems performing these simple checks.

You can even try running the Apple Hardware Test which you should be able to access by booting from the 2nd DVD which originally shipped with your laptop from the factory. I forget if you need to hold down "C" or "D" to boot Apple Hardware Test from that 2nd DVD (I think it requires holding the "D" key). Unfortunately the diagnostics are unlikely to detect a bad GPU, and it may not even detected a failing Hard Drive unless that drive failure is severe enough. With a third party battery installed, the diagnostic may report that third party battery as being bad even if it is perfectly healthy. The only useful diagnostic is one which reports a failure (assuming all original OEM components).

You can try installing macOS onto an external USB drive, but keep in mind installing to and booting from a USB drive on this old laptop will be extremely slow due to the very slow USB2 port, however, it is good enough for testing the laptop. If booting from an external drive works fine, then most likely the issue is with the internal boot drive (software issue, file system issues, or even drive failure), however, if you have trouble installing or booting from the external boot drive, then it may point to a wider system issue or possibly a Hard Drive with a very severe failure.

Creating a bootable MemTest86 USB stick & booting from it is also a simple way to partially test the system. While not as informative as using a full macOS external boot drive, it may provide a bit of insight. Creating & booting from a Knoppix Linux USB stick is an option as well since it is a full non-Apple OS running entirely from memory once it boots. However, the system may appear frozen on the Apple boot picker menu while Knoppix is booting so you need to allow Knoppix some time to finish booting....this was one of my favorite ways for testing older systems.

FYI, here is an Apple article for various boot issues....I'm not sure if these articles are still completely accurate for such an old system (I doubt it mentions using your OSX 10.6 Snow Leopard Upgrade DVD to get started with reinstalling macOS so you can install macOS 10.11 El Capitan)....you may need to locate the comparable articles on the Internet Archive's Wayback Machine.

If your Mac doesn't start up all the way - Apple Support

Edit: Booting from a full macOS installation on an external drive or from a bootable Knoppix Linux USB stick will allow you to actually check the health of the internal drive.

Thoughts. Since you asked, what was the tech's thinking behind his suggestion? No speculation, facts please.

More thoughts. Personally I would not invest a nickel in a Mac that old. As terrific as that particular model was in its day, and the fact Apple never replaced the 17" model with a display as large as that, it is of little practical use with anything requiring recent Internet security protocols. That means anything and everything Internet related (including email, browsing, App Store... etc) will present technological challenges that cannot be overcome due to the hardware limitations of its CPU, if nothing else.

Next, if you are bound and determined to keep using that Mac, personally I would not go through the trouble of reaching for a screwdriver to open its case unless you were to replace its hard disk drive with a SSD, and even then I would not recommend anything other than one sold by OWC / MacSales. Consider nothing else.

Next. That Mac shipped with two grey System and Applications Install DVDs that must accompany it from cradle to grave. If you no longer have, or can't find those DVDs, it's a show-stopper.

Finally. You wrote that you love that Mac. Love drives people to do all sorts of crazy things that make no sense. Working on that Mac makes no sense to me, but I'm not in love with it, you are.

According to MacTracker, the drive bays in that 17" Early 2009 MBP had 3.0 Gbps SATA-II interfaces, not 6.0 Gbps SATA-III ones. If you do get a SATA SSD to put into that MBP, this might be a reason to get it from OWC.

Other World Computing – OWC Solid State Drives For MacBook Pro 13", 15" & 17" (Late 2008 - Mid 2009)

when it does not start, what does it do instead?

what shows on the display, if anything? fans running? caps lock light operable?

I have a completely different suggestion. Rather than trying to find and buy obsolete hardware to save some money, do this instead.

If this is being used to support your work for a registered not-for-profit, consider purchasing the up-to-date version of Poser 13 and "gifting" the license to the not-for-profit/Charity.

Then deduct that cost as a Charitable Gift on your taxes.

Just get the Charity to write you a letter (on their letterhead) that they received this as a GIFT, (and did not provide any valuable goods or services in return) to cover you in case of tax audit. Or YOU write the list describing the gift and get them to acknowledge that description as a gift.

I'll leave it to you to conclusively determine the system requirements for that software, but the fact they make it available for the M series Macs opens a wide variety of options.

• Intel, M1 or M2 Processor

For example, if one were to interpret that literally, it omits the M3 or M4 series CPUs — the only ones in current production. That seems hard to believe. Apple no longer builds any models strictly conforming to that specific requirement.

Once you conclusively determine that particular product runs on current production Macs then I encourage you to move on from the much-loved 17" MPB and shop for its replacement on the Refurbished Mac Deals - Apple store.

If in fact it really, really needs a M1 or M2 CPU (again, hard to believe) there are plenty of them available on those pages.

Nice work, thank you!