I don't at all see how telling somebody they'll need two lights for a night dive comprises any kind of condescension. My AOW instructor told me to bring two lights to my night dive. It's pretty common practice, isn't it?
Sure, if you're teaching. Now, considering that Sean doesn't have instructor credentials, and there was nothing in the OP about Mark needing any sort of tips, hints, or training...
Even as an instructor, the only person I'd say something like that to was an actual student, unless asked. If we're talking about a fun night dive just as buddies, I would prepare myself and not offer that sort of suggestion unless specifically asked what s/he should bring for lights.
And truthfully... as long as you're sticking together, a shared backup is more than suitable in books.