iMac not starting up, black screen?

My friends 2015 27” iMac locked up yesterday, she switched it off via the button on the back and it now refuses to start up. She told me that the hard drive is about full, which probably caused it to lock up when she was clicking on stuff to try to get it to work.

Anyway, on booting up it shows the apple icon on a black screen, the progress bar edges along then stops and goes no further.

I tried safe boot but it won’t boot up in that either.

I’ve tried the recovery mode and I can get the options of reinstalling Monterey, etc but it won’t reinstall as it says the hard drive is indeed too full.

I managed to get it to make a bootable USB but when trying to boot from that the progress bar moves along with a clock countdown, it sticks at “about 1 minute to go” and won’t go any further.

Is the anything else I can do for her to get it started up to a position where she can delete a lot of stuff and get it working again, or does it sound knackered? Cheers.

That’s completely new to me, how do I do that please? She has a MacBook Pro but I don’t think it has an Ethernet port. How can I connect them in the way you suggest?

How long are you leaving it? sounds a bit like it's updating itself.