How do you help someone find their phone?

If the battery is depleted, there is no means to locate it remotely. No power means no radio communications and no location and no sounds. Searching is the only way.

If the battery is not depleted, it might be findable with Find My, or by calling out to Siri if that is enabled.

Here is what to do: If your iPhone, iPad, or iPod touch is lost or stolen - Apple Support

From that, and if there is battery power, the owner can use their Apple ID and password and Find My on iCloud: https://www.icloud.com/find

If (when?) you do find it, I’d consider acquiring a fluorescent protective case or such to maybe make next time easier.

And whatever link you’re following that’s looking for money here seems unrelated, unusual, or maybe even scammy. Apple does not charge for Find My.

There is no charge for using Apple's Find My feature, but if the battery is dead, you won't be able to find the phone using an online method. You will have to look for the phone the old fashioned way.

So, no sense in wasting time on something that will not work when the battery is dead.