Kana or not is beside the point in my opinion. It's still Japanese. Besides, if he really wanted to he could look up the kanji for the words. And as you suggest, it's not as if he'd have to be playing the games to the exclusion of any other source of Japanese. More exposure is always desirable, which makes not playing the game because of some specialized words a bad move, IMO.
By the way, I started learning Japanese with the aid of Dragonball Z. I know words like 界王拳, 太陽拳, 瞬間移動, 宇宙船, 惑星, 星, 滅びる, 滅ぼす, 全滅, 戦う, 戦闘力, 家庭教師, 所帯窶れ, 神殿, 民族, 全力, ~如き, etc. just because I watched the show (of course, some of them were re-inforced elsewhere). In that list are some words that aren't useful unless you're watching Dragonball Z or are having a conversation about it, but there are more words that are much more widely applicable to real-life situations.
All this is to say, I agree 100% with what
@JimmySeal said.