I don't believe there is any adapter out there for an RCA -> speaker inputs on a sub. The issue is that the different inputs receive different signals. On newer subs, as you know, there are two inputs. The RCA jack (which is what the coax was run for in your friend's house) is only for bass signals...the receiver has a built in crossover and only sends signals under that crossover point to the sub. On the receiver, it's the sub pre-out. Also, that will be a non-amplified signal because most subs have built-in amps.
Conversely, the speaker level inputs on the sub are designed to receive the full audio spectrum because they will also be feeding your main speakers. If you use these, the sub will be using it's own crossover to strip out the bass signals and sending the rest to the speakers. The signal received from the receiver will be amplifed and full-spectrum.
So, as you can see, both those inputs are for very different things. Your friends options are (1) upgrade to a newer sub or (2) run speaker cables to use the sub's speaker-level inputs. If your friend chooses option (2), keep in mind that you'll have to run speaker cables from the receiver to the sub and then from the sub to the speakers.
Hope this helps.
