I was once put in this position and decided I needed to let my friend know. But she found out before I could tell her and when she found out that I knew but didn't tell her she would not speak to me for several years.
The bottom line is you don't know for sure; it's 2nd hand news.
I'd suggest you tell the b/f that there's a rumour out there about him and leave it up to him to sort things out. If it's happening, it's his mess.
Tell him you'd hate to see your friend hurt if the rumour gets back to her.
You are not obligated to say where you heard it from.