How do i deal with my best friend?

she is 19 and her mom and her step dad got a divorce about 2 months ago and her step dad wanted the divorcee and my best friend lives on her own and i was over to her place and there was a knock at the door and it was her step dad and they kissed on the lips and they sat next to each other and he put his hand on her knee and after he left i talked to my best friend and she said she has been dating her step dad for about 7 months and she told me she was 3 months pregnant and he is planing on moving in with her and her mother doesn'tt know anything. so my question is how do i deal with this my best friend dating her step dad. and should i tell her mother whats going on? my best friends mom and now ex husband have 3 kids together. so will it be awkwardd when they have family dinners? please help me i don'tt know what to do.