Native English speakers would probably ask for clarification! If it is Monday, May 11th, and he's coming back the 15th, I'd expect him to say "I'll be back on Friday" or "I'll be back this Friday." For the 22nd, I'd expect him to say "I'll be back Friday of next week," or "I'll be back Friday, the 22nd." If he said "I'll be back next Friday," I'd ask "You mean the Friday after this Friday?" or something like that. If I COULDN'T ask and had to guess, then, yes, I'd guess the 22nd.

Oh and 'following' Friday and 'next' Friday can mean the exact same thing. Just different word choice.

Honestly, a lot of native English speakers don't even know the answer to this XD I always ask for clarification, by asking "Oh, do you mean the 9th or the 15th?" It can be confusing because the person could be meaning the Friday that is THIS week (which would make sense as the 'next' friday) or they could mean The Friday of the NEXT week. So if you're unsure, it isn't unusual to ask for clarification :) No worries.

For most people, 'Next Friday' would mean the Friday after this Friday, meaning the 22nd. I believe 'the following Friday' would mean the same thing. If he was coming back on the 15th, most people would use the term 'this Friday'.