vaporisas said:
Don't confront him, unless you are with someone, that way his anger level will be moderate and nothing stupid will occur.
However, the best way of going about this is to consult with you mother. Since your mother has been hurt before, it would be wise for you to not be too blunt about the situation from the start. When she comes back, ask her how her trip went, if everything was fine, etc.
Afterwards, and make sure you got her in a good mood and little by little give hints as to what you saw/know your father is doing. Don't say "dad is cheating on you" because it will be too harsh and could cause her to go through such an emotional downfall that repercussions to her health or rationalization at the time might not be what you will want to occur.
Little by little, let her know in a calm manner that you suspect that your father is perhaps cheating, etc.
This is a serious situation to be involved in, but I wish you the best of luck in dealing with this.
Remember, sometimes people (and this is a fact of life) make decisions that are completely irrational in all senses. Why do people smoke when they know it kills them? Why do people fight in front of kids when they know its bad for the kids? Why do people do irrational things?
Easy questions with no clear answers.
I wish you the best of luck in this situation.
Sincerely,
-NAL