Hi there and thanks for this question. I am Dr. Linda, a vet with 10 years of clinical experience.
How much sexual interest a male cat has in a female, really depends on them. If they were neutered at a young age (before about 6 or 7 months old) they generally have not developed sexual habits and should show little to no interest in a female, even if she is in season.
However, if the male was only fixed recently and has previously mated a female, he is more likely to potentially mount or mate with her. Of course, there would be no pregnancy.
As she is now 6 months, we could have her spayed at any time and I'd recommend having this done soon, so she won't be coming in and out of season.
Until then, I'd keep them together as normal but keep a close eye. If he does start to act inappropriately, we may need to separate them temporarily. It may also help to keep his mind on other things like food puzzles and interactive toys.
If you do not already have one, a Feliway Friends plug in can help create some more harmony between them both.