too late

I know someone who believes her cat is pregnant and I need to know at what point is it too late to spay your cat during her pregnancy?

IMO its morally up to you and also how late will the vet spay with kittens.

If I were faced with the situation, I'd spay up to 4 weeks pregnant. The problem is you don't know if they are pregnant till 3 weeks pregnant if you've seen them mate. Maybe 5 weeks would be ok - they would deliver in about 8-9 weeks anyway so you would have to go by what you thought was best and the vet's advice.

Actually if she calls around different vets do it at different times.