Do i tell her hes cheating

So im friends with this girl on fb. We dont know each other hardly at all and i dont even know how we ended up friends on there. We know each other through mutual people. Well anyways she had posted awhile back about how her bf cheated on her in her own car. It was a really upsetting story about how he fucked some girl while she was at work and the girl had bled all over her seats. She broke up with him for a week or so and i see that they are back together now. Well like less than a week ago i saw him at walmart from a distance, being really laughy and giggly with another girl... obviously not his gf..getting into a car and driving away. Now i dont want to say anything because 1. I dont want to stir shit up 2.she already knows hes a cheater and she took him back to begin with 3. What if it was completely harmless and the girl he was with was a family member... i mean i doubt that last one but you cant ever judge a book by its cover.