Good or bad

Date a man for about 2 years and knew nothing would come of it. He walked out of relationship without a word. Got to know his kids through it. Now he has begun dating his sons girlfriends mother. The boy and girl have been dating for 5 years. As of today the boy and girl have gotten engaged. First, is it a good idea for the mom and date to date each other? Do I tell the mom the date is still on a date site? Do you think the mom and dad dating pushed the boy into getting engaged with the girl because he felt pressure from the mom and dad relationship? Thanks.