"What is his problem?"
Translation:Quel est son problème ?
how can "what is his problem" be translated to: qu'est ce qui lui arrive?
It's a common idiom. Litteraly "what's happening to him". I'd say it's the most natural way to translate this English sentence.
In real-life French (because I guess Duo rather teaches textbooks' French) "Quel est son problème ?" can sound a bit patronizing, like you're implying the person has a problem because he/she is stupid (I wonder if it's not the same thing in English though).
Hope it helps
Qu'est-ce qui lui arrive sounds to me like a proper translation of what is his problem. Maybe report it! I'm native.