MacBook does not power on, but does chime.

I have seen this a lot when the device is encrypted with FileVault. It can be really touchy about outputting to an external display until the disk is unlocked. And there would be nothing on the touchbar either. First thing I would try is just close the lid. See if it forces the display to the dock output.

The other thing you can try is sort of blindly logging in (assuming you know your friends username and password). It’s best to do this from a freshly powered on state to avoid any changes to selection that may have been made. Once the device is on, press either the first letter of the user name of the account, or the right arrow key to select the first account in the list if you aren’t certain. Press return, type the password, return again and then hope like the dickens.

You can also try forcing to external display by simply disconnecting the display cable.

I would have a look in this area anyway, since a small bit of liquid damage in the display connector could have sent backlight voltage into CPU data lines for display. It’s an all too common issue. And that would be a new logic board sort of deal, since replacing the CPU is not economically viable for the sake of repair.