The fact that she's been ignoring the MC for 2 weeks and all this time she's out and about having fun with some random dude doesn't count as an action?Ever hear of two wrongs not making a right? You simply cannot know she's cheated. So it wouldn't be a reaction on the MC's part... it would be an overreaction. And it's not like she sent that pic to you. Someone else happened to see it and happened to send it your way. That's not what you do when you want to "fire a shot". That's like setting a bomb in the downtown bus station bathroom in case the dude you know takes the bus to work might need to take a piss and go in there.
We saw just one picture, but we found out that he's in multiple pics with her and I don't think she's been going around capturing sad moments.
Also the dude can't be a close friend or relative because the MC would have recognized him. They've been living together for a while, there's no way he doesn't know her friends.
What should the MC do when she's not picking up the damn phone? Wait for her to grow up? She didn't send him the pic, but she's been posting multiple in her social media. She knows what she's doing. She's either fucking someone else or she's trying to make the MC jealous. She should've known that something like that can backfire.