Let's think about it like this. You got your friend with an ability that's unknown to everyone in the kingdom even with extensive training and research. But your friend still wants to try and help with the team. The right play would be giving the friend a work behind the scenes role in the team, which not only gives him more time to try and find his power, but also keeps him safe. But instead, Sasha decides to act all cold and berate the MC for being useless in a attempt to get him to quit on his own when its CLEAR he will not do so because of the dream he holds on to become the strongest together.
So he goes in with the team and almost gets murked off. Instead of consoling him, Sasha decides to berate him even more, creating the small rift that will over time start to grow more (you could argue that MC not knowing when to back off is a fault of his but again, there are better ways to make him valuable to the team without putting him in danger).
Now that things have come to this, its clear that your friend can't keep up in the field anymore. So a decision has to be made to keep him out of danger. Some good options would be to either give a desk job or if absolutely necessary, tell him that he needs to take some time off from the party to work on his abilities and maybe unlock his special power but say he's still welcome. That way, they can still be on good terms.
What does Sasha do? Keep up the cold act and cut him off when he's about to say something, and turn it into a full on banishment from the party in an attempt for "His own good" KNOWING that he probably won't quit due to the dream he still carries. So relations get broken off on a very sour note and the rift between MC and Sasha grows even further.
Fast forward a couple of months, friend is reduced to D rank, having to collect herbs to get by, all the while with distasteful rumors spewed every time he steps foot into the guild. You see him and want to give him some help then recall a place you saw on your travels that had a rare herb that could probably be of some help. Knowing this, the average person would go to look up information on this place then go back to the MC to give him information about the place if it was safe and strict tips if it wasn't.
What does Sasha do? Proceed to just tell the MC about the place while not bothering to crosscheck recent information about the place and send MC on the way, while only JUST finding out about the true nature of the place along with the rumors that've been circulating around that weigh on the situation that SHE caused in the first place.
So, a couple of minutes later, a commotion is heard and (suprise suprise...) your friend comes back with his right eye gone and a ton of wounds on his body while having with him the head of the SS+ monster that tried to kill him. He crashes out, believing that you tried to have him killed which stems from all the rumors that you inadvertently caused and has decided to cut everyone off right there and then before passing out, furthering the rift between you and him into a full chasm. (keep in mind, all MC has seen of Sasha's character recently is just her being cold, so the moment she was being nice ends up leading to him almost being killed, how else is he supposed to come to terms with how to feel about Sasha and everyone else?)
Now things have spiraled out of control. It seems your friend has awakened his ability but at the cost of injuries and a major misunderstanding that has broken your friendship apart and can potentially degrade even worse if left unchecked. Now, a normal person, hell someone with a BRAIN would think to wait until the friend wakes up and then apologize right from the get go while telling them everything to salvage what's left of the friendship.
What does blockheaded, naive, 14-year-old Sasha do?
Proceed to run away from the situation entirely by accepting a mission far away from the MC and avoid him. For 2 years. And then, and only then, when she catches him at the S-Rank congratulation party, decides to say sorry while fully expecting him to understand her actions.
I don't think I have to explain the repercussions of doing that, as you can see with your eyes how that made MC turn out.
But it's not like you can chalk that up to bad writing either because as mentioned before, Sasha and the MC were 14. So while you can argue the outcome could have been conveyed better, it is not unplausible for all of this to happen since at the end of the day, they're still kids.