If you are on the car title as a co-owner, you should have an equal right to possession of the car. Check with the local police to confirm your right to possession, and so they don't treat the car as stolen if you retrieve it. It is unlikely that the police would assist you in recovering the car, but you can ask.
Once you have the car back, if it is titled in South Carolina, the easiest thing to do would likely be to return to South Carolina and follow that state's laws in relation to exercising your contractual right to sell the car. You may have to get a court order, given that the other owner is not likely to cooperate in signing the title.
Your friend could theoretically sue you in either state.

