[Zoro is one of the few people that Reno would truly call a friend. Sure, they fuck from time to time, but mostly Reno values the times when they just sit together and talk. Or even don't talk and just nurse their respective drinks.
He's told Zoro some of his thoughts and problems and so did Zoro, so .. yeah, Zoro is the closest thing to a friend Reno has, since Rude basically ranked up to lover.

He walks besides them, hands in his pockets and still grinning all over his face, until Rude suggest he should be the camera man.]

Oi! No, I will not? I'm playing the student who gets his hot teacher, remember?
Also you couldn't expect me to stay out of it, if the two of you were getting any closer than right now, that's for sure.