G.B.
Tell her the doctor says he is having a really bad reaction to the fleas and he can't be around animals anymore. If she just doesn't get it then you have to be blunt. I am a bit dense sometimes and need to have things pointed out in black and white...LOL, so maybe you could just tell her the doc said so and leave it at that. She shouldn't be mad at you that way.
She doesn't care enough about him to put flea medicine on her pets. I say give her notice that you are going to change him and she has...for example, 1 month to comply or you have to move him for health reasons.