I wouldn't even talk to Katie about Behira, at least for a while. Don't even bring it up, none of this talking about 'things went so well' to Katie, Katie may just dig in her heels even more and push for getting rid of her. Give it time. Don't push anything, don't even talk about Behira. Clean, handle Behira and do your thing with her not involving Katie. If Katie goes and takes her out or does anything with her, great, but don't go on and on about it, it'll probably annoy Katie. Focus on the others, pretend Behira isn't there, she's not a bother, that kind of thing. Perhaps in time Katie will loosen up about it. If not, well, you'll need to rehome her. It's a sucky situation, I've been there and I've given up reptiles for SO before, it's never a good position to be in. I hope things work out.