Just tell her but there's a chance he might have to fight underwater at some point so this is sort of like training for that. Also, tell her that all her friends will leave her if she doesn't wasp and she won't be able to get a mate as well. Or have her watch that one episode of The Powerpuff Girls where Buttercup won't take a bath and even the monsters won't fight her. Also if she goes to the real world like that she could get sick and die. If this might be too distressing to her and just promise her catnip, cuddling, and sex if she does. You could also just give her the nickname "Stinky Sarah" that might do it. Sarah Blackclaw sounds like a good name for a Gatomon/Tailmon.
Just tell her but there's a chance he might have to fight underwater at some point so this is sort o
