So long as he's not trying to agressively monetise it, it's just fan work, and it should be fine. Japanese creators and publishers are usually pretty chill about small-scale doujinshi manga at conventions and such, as well as fanart sales.
If he tries to monetise it though, and it gets big, all bets are off. Using copyrighted characters can be a legal quagmire, and a crossover is unlikely to fall under the blanket of 'parody' so he could find himself DMCA'd if it became big enough to attract the notice of the copyright owners.
I doubt he'd find himself in much legal trouble, but there is a possibility. Most likely, he'd just be told to take it down, which would be an emotional blow if he's poured loads of time into it.