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).