Do you think once a cheater always a cheater, or do you think they can actually change?

As long as a cheater knows that they can get away with it they will keep cheating. If you can give them a reason not to cheat on you, as in ask them what is the thrill of cheating? If you can provide that thrill then you might be able to convince them not to cheat.

Both men and women can and do cheat in a relationship.