Well, all considered if your friend uses the same password and/or username for her PMC and/or anything MC related and it isn't working... well hacking is a huge possibility. Although, if not it depends then, it could also be a computer virus or malware. I'd say if she has any anti-virus or anti-malware (anti-malware is MUCH more reliable in my opinion though ^_^) she should scan her PC just in case . If all else fails though, she could always contact Mojang/Microsoft and report her account hacked if she still can't log in. As for advice I'd say that's the most I can give at the moment but, if I think of anything else I'll edit and add it in here. Hope this helps!
Edit: Grammar fixes