What should I do if I find dad having an affair with someone else?

Sit tight and see what effect your information will have on your father. Your father should spend most of his life playing around and living a good life. The present life may be a little tired. It seems that the other party also has a family and should not run away from home. People are often lucky. If it is caught, it will stop. If it gets worse, you privately warn him to go on like this and let him leave home by himself.