Taht's great news!!

I was about to ask though, WHERE was the son petting her when she tried to bite him?? He could have been petting a sore area on her, she could have been sick or hurt or injured! A dog that has been as well-behaved as implied in the original message doesn't just bite for no reason unless they have a reason to... have they brought her to the vet lately to get her examined??

You mentioned she has arthritis... the child could have been petting her and hit a sore spot, and she reacted that way because she was in pain. It's very common, and she isn't biting for "no reason" in that case.