I dont know how i should do about this situation?

ok so about a month ago my mother and step father said they were getting a divorce and that they would try not to make any more painfull than it already was, ok so now about a week ago my mother say's she wants to work it out ok but one afternoon i walked in on my mother talking to some guy, so i guess becuase she felt guilty so she called me in her room and thats when she told me that she this dude on my space and that she has feelings for him, my firts question was what was wrong with her and how could she do that to my father i mean he has been there for her he took me and brothers and sister like we were his own he even planed to adopt us this summer he does what ever she wants him to i mean he dosnt go out he go's to work and comes home every day and helps take care of herhe 2,3,4 o'clock in the morning with her becuase she works from home and then he gets up at 5am and gos straight to work i dont if i should tell him or make her tell him her self but i fear that she wont be honest and im confussed on how i feel i mean if i had to choose sides i would pick my dads is that right o wrong?