The right thing to do is for the computer-owner (who knows the passwords) to follow these guidelines, then install ANY old version of MacOS:
What to do before you sell, give away, trade in, or recycle your Mac - Apple Support (IS)
Because you do not have the passwords, YOU (as non-owner) can not do these steps for most modern Macs. This is deliberate, to prevent you from re-using a computer that was stolen.
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if the display will not light up at all, you can connect an external display or TV set as a work-around. With an external display, keyboard, pointing device, and power adapter connected, closing the lid on the MacBook Pro should light up the external display.