If he's willing to try and communicate in English I think it's fine even if his skills aren't so good. On my server we have an issue with non-Japanese-speakers who refuse to listen or communicate with their DF groups at all even though they flagged Japanese (since we're profiling, they're always English speakers; the French speakers I've met have all flagged themselves properly and been very sweet). These non-communicators are universally terrible. But just making the effort with some auto translate and a few clumsy words can make a huge difference, and most groups will try their best to form an understanding across the language barrier. I personally dislike people who flag languages they won't even try to speak as it means everyone playing properly in their second language will experience more prejudice as a result - that's my opinion as a minority in my datacentre rather than the dominant group who only want to speak in Japanese. I also hate completely silent runs as they're boring. Being able to chat with my party members and encourage one another makes everything flow better.
Incidentally, I believe there are two different sets of language preferences which is where the confusion is arising in some posts. The language flags in your search profile are configured in your settings, and denote the languages you are comfortable speaking. The language flags in DF are only used for matchmaking in DF, and they denote the languages you wish to play with for that particular activity. This is, of course, needlessly confusing, but I definitely only flag J for DF yet I know my search profile has me correctly flagged with all of my languages. I've been in parties where someone is listed as E-only but the JP players are asking one another how they got matched when they only flagged J because they hate not being able to talk to their group. The reason is that the E player flagged J too despite being unable/unwilling to communicate in that language. Niwashi's suggestion would definitely make it less confusing, though it might lead to (probably deserved) witch hunts when people outright lie and draw attention to it by refusing to communicate.
While we're sharing tales of passive aggressive Japanese hostility, I've never seen it where it wasn't justified but the funniest incidents are always in FL. Someone accidentally said the Japanese equivalent of 'good game' once halfway through a match by fumbling their chat box and the harassment kick message was almost instantaneous since they were perceived to be lowering morale and whining, lol (I voted for them to stay as they were cohealing with me and doing a great job). Another time I had to report an English speaker for opening a FL match with a sexist slur because he presumed nobody could understand him on my datacentre (sucks to be him). Most negative experiences usually involve lots of attempting to reason with the bad player in multiple langauges and discussion with the other Japanese-speaking group members before any action is taken, though. The glimpses of drama on English-majority servers are something else.