So I've been friends with this girl who's younger than me for all my life. The reason we've known each other for so long is because both of our families have been close family friends from before we were even born. Her name is Brianna and she's dating this guy named W (I'm not going to say his name because it's an Arabic name and I don't want them to see this so from here on out, I'll just refer to him as W because that the first initial in his name). Brianna and W are a really cute couple and I've even hanged out with them in the past. They're really nice people.
Earlier today, my friend who's gay told me how he saw W on this gay hookup site (I think it was called squirt or something like that?). I asked him if he was sure that it was Brianna's boyfriend and he said he's sure because W's full name is his username on the hookup site. I told him to send me a screenshot of his profile and I checked and it really is him and he has the word "Bi" on his profile.
What do I do? I've been friends with Brianna for so long and I've always been honest with her and I feel like she should know about this but I don't want to ruin her happiness. They're such a cute couple and the last thing I want to happen is for Brianna to start being depressed and stuff like that. What do I do? Please help me out.