This answer is going to kind of suck, but it is the truth: He will be able to play League, but if he continues to get AFK penalties he will eventually have long wait periods as he rises in the tiers. (We aren't banning him from the game, but he will be unable to play for 2 weeks at a time at the highest tiers)
There isn't a great way to detect the difference between a crash and someone doing it on purpose (and if there were, people would just cause their game to crash to defeat such detection). The priority call has to be to preserve the experience of the most players possible.
You probably aren't winning a large % of the games where he goes AFK, and that can't feel great to your teammates either. It also turns out it doesn't feel great to the people on the other team who go from having a fun back and forth to a hollow victory.
If he hasn't already, I would recommend reaching out to player support to see if there's a common resolution to his crashing...
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