HelloCustomer I hope that was not a friend, playing a joke. I would definately not start it. I can't tell you it would definately damage the motor, but it's not worth the risk. You should have the fuel tank removed, drained and cleaned. Since you haven't started it, no tainted fuel has flowed through the fuel pump and into the lines yet. It is cheaper to have the tank removed and cleaned, than replace the fuel pump, and all of the fuel injectors. Whatever you do, do not turn the ignition on. When you turn the key on, the fuel pump runs for 2 seconds. Try to avoid that at all costs. After the tank is clean, have a new fuel filter installed and you should be OK. I've heard of a lot of things but into fuel tanks, but I never heard of mothballs. Good luck and after this, buy a locking gas cap just to avoid a repeat. Let me know if you need any help.
Your JA Expert,