I can see why the MC's ex moved on, 5 years is a long time of mourning and yearning, and there was an opportunity for someone else to come and console him and feel the void. Especially since he was feeling all the heartache and the pain of not knowing where his loved one is, and also the flashback showed that ex had asked MC to stay with him and not go out and about, which MC did anyway (to which I say, go follow your dreams, you only live once and all that). And, the ex might be bitter about that: MC left him even though MC said he loved him, and he did not know where he was, how he was for a long time.
BUT...a big BUT, I am honestly kind of amazed at the lack of empathy of the ex. Bruh, MC has been in a coma for 5 years, and finally returned home--his reality is completely different than the ex. For all MC knew, he has always loved the ex. And I find it quite amazing that the ex does not even acknowledge that. The ex got 5 years to get over him, MC does not even get a second, until he is confronted with the one he loves being with someone else with no explanation. Just imagine the hurt, seeing someone who you love wholeheartedly be with someone when you are coming home to them. I think it's fine to give up on someone, especially since people change. But the reaction of ex towards the MC irritates me, as he shows no empathy after his initial anger and frustration. He is just like "Yeah I moved on and this is my partner." The ex could have taken a second and thought about the MC's perspective, and delivered the same message in a different manner or something.
I was totally impressed with the main character for confronting the actor dude about the kiss and his feelings and not secretly pining. And then we got that shitty response from the actor dude...