My b.f.'s g.f. cheated on him & got preg. she is my firend too, he doesnt know, what should i do? please hlp?

this girl is my friend too! i dont know what to do! he doesnt know she is pregnant but he knows she cheated on him. can someone please give me some advice? she knows that i know but hasnt told him. she said she might tell him, but what if she doesnt? am i supposed to just sit here and watch!?