Good advice, get an attorney and shut up.
The attorney will probably figure out what is going on -- you need a flow chart to figure out all the potential causes of action.
1. The mother probably has a cause against her son if he did not pay her monies owed (car loan).
2. You probably have a cause against her son if he failed to pay back monies owed to you (car loan).
3. The son may have a cause against you, depending on the nature of your business relationship.
4. Mother probably doesn't have an action against you for her son's car loan (no privity of contract).
5. The mother may have a cause in quantum meruit . . . however. . . quantum meruit usually involves something like the following scenario: you pull your Airstream into an RV park and blow a tire. Unbeknownst to you, another Airstream in the RV park has a blown tire and calls for a serviceman to repair the tire. Instead of repairing the tire of the other guy who called the serviceman, the serviceman repairs your tire. Because it was through no great fault of the serviceman, the serviceman could sue you in "quantum meruit" for the value of the services that I performed in repairing your tire.