Can the cosigner come after me?

If I understand you correctly, a friend of yours co-signed a loan on your car.  You are asking if you trade in your car, can you leave all the debt to him.  There won't be any debt.  Your lender has a security interest in your car and it has no doubt followed standard practice by recording that lien.  You won't be able to transfer title to the car until and unless your lender consents, which will not happen until and unless the loan is paid off.  Its just like a mortgage; the mortgage is recorded, and the house can't be sold unless the mortgage is paid off.   If, somehow, you managed to trade in the car without paying off the loan (I don't see how that can happen unless you commit fraud), your cosigner will sue you for any payments he has to make on the car loan.  On these facts, he will win.