If a married man had one mistress in his whole life who he was friends with but doesnt see anymore because he got caught cheating if he cheated again would he go back to her?

The situation that you describe suggest that the married man and his mistress have mixed feelings. This is not unusual. They can see reasons why they want to continue their affair (perhaps the sex is really fun) and they can see other reasons to end the affair (perhaps to avoid damaging the man's marriage or causing him to pay a large divorce settlement).