Could your friend take Purdy as well? I must say the thought of splitting my two 18 month old cats up would be heart breaking. They are so close, they do everything together. I worry about something happens to one of them as I know the other one would be so lost. Maybe yours weren't that close, I've heard other people say their cat grieve for the loss of other cats in the household even when they aren't that close, as they just miss having the other cat there.
I think you will need to give her lots of extra attention, plays with her and give her lots of treats. Did they play together? She could be missing that as well and might be getting bored.