Help needed, boyfriend owes more money - What would you do?

Yesterday I got an email from my boyfriend's business partner, I'll call "R." The two of them are working on several projects that will (in theory) make more money. Except right now my boyfriend owes R $300. And R is fed up.

To quote R, "I've grown tired of being our bank as I'm sure you have too.  I've grown tired of being the only adult one in our partnership."

Welcome to my world, right? R wants or expects ME to intervene and help him get his $300. (Ironic, considering how much my BF owes me for carrying our household...) But I don't know how to bring it up, or if I even should. I certainly understand R's frustration! And on one hand, if my boyfriend doesn't shape up, the business partnership might fall apart completely, who knows.  **I should explain that the business partner and his wife are our friends; we do things socially as well as regarding the guys' business stuff. So that makes this even more touchy. R and his wife make good money from what I understand, but that's not the point. My BF owes them money and they want it, now.

Last night I tried to vaguely bring up the subject of R and their business "stuff" they are working on, but I didn't ask bluntly about any money being owed. I chickened out. I don't know how to bring it up without being super blunt. Maybe I should be?

Any suggestions would be great. Thanks so much.