Why wouldn't he just offer you a Captain position if he is your close friend? Especially if it sounds like they are looking for a Captain as well.
Something does not sound right here. A contract would be helpful, but as previous poster said, with the very rich (especially in place like Taiwan or mainland China) contracts are worth the paper they are written on.
If your goal is a US legacy carrier, then I would stay with the airline unless the situation is really bad or you know you will not upgrade for a long time.