Have you had a professional check to see if you do in fact have fleas? That is the first thing I would do, then if there are fleas I would have the exterminator treat the home. If the guest refuse to have the home exterminated while they are there have it scheduled for their departure date. I would give them some type of compensation if there are fleas. If the exterminator cannot guarantee that they can be eliminated prior to the next guests arrival I would contact them and let them know they have the option to cancel with a full refund.
Years ago I had a guest who stayed for an entire week and then demanded a refund due to "bedbugs". They turned out to be professional scammers and took vacations all the time on VRBO owners who were unaware of the scam. The picture they sent us of the "bites" was a picture taken from a medical wiki site.
My opinion is that if your guest has complained of fleas, they need to allow you to treat the condo immediately. I would not care about "allergies". Have them stay elsewhere or have them leave early and refund for the nights not used..