Well, cat dad was a bit naive for sure. But there are no perfect answers in a situation like that. And you know, six year olds don't get to decide anyway--she wouldn't have gotten to decide anything if she was with her abandoner-parents, either. All in all, taking the memories away for a while so she can move forward might have been the best thing--I feel like that might actually be a good treatment for some traumatic stuff. But he should have had a plan to give them back.
The time thing was maybe not so great. If you're gonna give a human immortality, wait until they're in their 20s or something (and ask first, but I'd sure be up for it).