I was in a similar situation earlier this month because a guy I'd liked on/off for over a year decided to confess to me that he had liked me for a while after he got drunk one night. However, this guy had dated my friend before, and is very well known for having a lot of casual sex.
Anyway, I'd recommend to just think for yourself for a minute. What do you want? Are you after a relationship?
Ultimately, consider this; would you risk your friendship with C for a relationship with R.
It doesn't have to be a big deal if you do decide to pursue a relationship with R. Talk to C about it, and explain the situation to her. Tell her that you understand if she's upset with you, or if she doesn't want to be with you when you're together, or even that she may need some time to process things as they stand, but make sure she knows that she is an important person in your life and that you don't want to compromise your friendship with her over a boy; make sure she knows how long you have liked him and that it's not just a 'fling'.
Personally, I would have pursued a relationship with the guy I liked, if I wasn't so inexperienced/scare of his past/happy being single/applying to a different universities to him. There really was no future for us, I guess.
If you seriously like this guy, I'd say you have every right to pursue a relationship with him - you will always regret it if not. Remember, friendships don't last forever, but if C is a true friend, she will forgive you.
Relationships are a sticky mess eh.