re: Last night i found out my 68 year old dad has a mistress...

Any idea what caused the cheating? Recently retired? Dementia? Any changes in life? I'd agree this likely either isn't the first time or something likely set it off which could be dementia or another mental disorder from aging? I'd discuss that with your dad. If it's not his first, that's a different story and your mom may be about to go through hell.