My friend. He has a girl and a son. BUT has this other girl and son who he sees but his gf dont know!?

one of my closest male friends has a gf and a 2year old son....but then there is another girl who has his child (1year old) and he still sees the 1 year old AND the mum. but his gf has no idea he wont tell her and lies about them. she heard rumour that he had another child but he lied his way out of it. she thinks he's with her and only her. its really not the case. his relationship with the other girl is sexual and feelings are involved she does know about the gf but doesnt make a diff to her dont think dont know but he needs to do some thing while he can man coz i hate standing round watching all this go on. i know both the girls but dont get involved. why cant he just explain everything or be with one of them? he dont really talk bwt it with me that much jst says dont worry. grrrr why u think hes doing this? wat should he do?