Search Results: giving him the wrong idea

My best friend, "Emily" has been with our mutual (close) friend, "Mark" for 3 years. Mark is a saint: Emily has aspergers, a personality disorder, and a 4 year old daughter from a previous relationship, so suffice it to say, being with Emily is hard, but Mark is an amazing influence and he is probably the best thing that has ever happened to Emily and her daughter. Mark is completely supportive of Emily and has been helping put her through community college after her abusive family cut her off financially (which means Mark had to take on 2 jobs on top of school work) and he's been like a father to her 4 year old.

Mark adores Emily, which is incredible she's NOT conventionally attractive (obese) and until recently I thought she adored him too (which is how she acts when he's around, in her own way). But, she's been talking about this guy from school, "John", in front of me and Mark for a few months, joking about how everyone thinks that they're dating and how preposterous that is.

Yesterday, I was in the wrong place at the wrong time and saw Emily engaged in oral sex with John, while Mark was at home taking care of Emily's daughter. Worse still, Emily gave Mark the excuse that she needed to go to the party with me (without her daughter) because I've been having a hard time lately, so I feel responsible for what happened even though I had no idea that John was going to be at the party or what was going on.

Emily is ruining the best chance she has of giving herself and her daughter a happy and normal life. Mark is such a great guy that I feel morally obligated to tell him what I saw, but at the same time I feel that I need to protect Emily and her daughter (ESPECIALLY her daughter, who thinks of Mark as her dad). What should I do? Is it too late to make Emily see that she is making a big mistake?

TL; DR: caught my best friend cheating on her fiancé who has been raising her daughter and putting her through college (and is also my friend). What do I do?