I don't need to read ahead to know what'll happen, don't worry.
I honestly would be fine if he wasn't focused on "revenge" toward her in that way. Just outright ignoring her and his former party members and doing his own thing would be kinda ideal - then, if any of them give him flak for it, he can just say "you considered me weak and kicked me out. Why are you mad that I'm not helping you now?"
That forces them into the position of either having to solve the problem themselves since "they're so strong and high-rank and he's just the weakling they kicked out", or - and this is the operative bit - they have to swallow their pride and ask him for help.
It's one thing to swoop in and actively save the day to show up his former party members/childhood friend, but making them come to him for help themselves has that extra layer of shame.
I've not actually read the novels yet (though all the griping via spoilers here makes me feel like I have to an extent), so I don't know who you mean - but I'm sure we'll get there eventually. Mostly when I say "stay a loner" I mean "don't fucking go crawling back to the authors waifu", so if there's other awesome characters in the pipeline that aren't affiliated with Sasha & Co, then I'm down to get to that bit of the narrative whenever.