Girlfriend cheating?

So my Best guy friend and I have been super close for about 7 years. We been living together for 3 years now. This guy is literally my brother.

He usually comes to me with any his relationship problems the same way I vent to him about mine. We usually talk about everything and anything.

Today he called me drunk and upset and told me that him and his girlfriend of 7 months were arguing. He said she just started acting weird out of nowhere and started ignoring his calls and wouldn't tell him what's wrong along with sending him short replies, He told me she started posting pictures of herself in her Snapchat in a bra and tiny shorts.

(She never does that) and she has hugeeeeee boobies so they were taking up the whole picture👀🍈🍈

He started freaking out asking me what should he do and I honestly don't even know what to tell him. He logged into her Snapchat and a lot of guys were replying to THAT picture with dirty talk and she was going along with it.

So my bestfriend was trying to figure out why she's acting like that towards him and I was thinking maybe she's losing feelings but Noooo HONEY.

He ends up having this conversation with her bestfriend and she tells him that she's upset because OF ME. 🙄

Like what the actual fuck.

According to her she heard my voice in the background whenever they were in the phone and that got her upset, The fact that we're roommates.

I feel so hurt because she KNOWS there's nothing going on I love that dude like my own brother and I've never seen him in any other way. I dont know what to do or what to tell them. I have a boyfriend by the way and he's even super close to my best friend they get along super good, my boyfriend spends the night almost every weekend and she knows that. I don't see why she wants to act like that now.

Any advice?:/