his posts are being flagged
Maybe he flagged it on his own? Unlikely, but possible
On the not-so-serious side:
It’s the rumored 
PM Police where the bots of their department try to flag any and all foreign posts in PM. That’s out of my league, I definitely don’t have anything to do with it.
Police Police
The only way for it to be flagged is by either you or him flagging it, or simply some kind of security measure on the forum flagging it. It might be because of the use of a different, unknown language or something. Whatever language that may be. Try talking in English, perhaps that will fix it.
Hey, I answered via the same PM. I have not noticed this behaviour before. Is your friend a new user or on the same IP as you?