I guess I'm kind of confused, but honestly I don't understand why you are needing to run task manager so much, or at all. Especially if this is your parents PC. The only time you should NEED to run task manager honestly is when you're trying to troubleshoot some other kind of problem. If task manager is the only problem you have, and it only crashes when you manually run it, then, don't. Why do you need to anyhow?
I understand that it should work, and that it shouldn't be a problem, but the work around or something about your hardware that isn't compatible with the requirements of Windows 11 is probably what is causing this and if you don't have this problem unless you intentionally open the task manager, then not running the task manager (Which like I said isn't something most people will ever likely need or want to do anyhow) seems like an acceptable trade off for getting 11 to run if you insist on it. Technically speaking, there are reasons why older hardware isn't supposed to be permissible with Windows 11, and while I agree it's kind of lame, it is what it is.
Is this a legitimately licensed Windows 11 activation or has that been "worked around" as well?