Could they obtain a licence and start building A320s in Renton? I guess that depends on how hard it is to build the tooling. Or AB pays BA to build A320 to cover demand?
This kind of thing is not unheard of in the electronics and semiconductor industry, that powerful customers forcing a temporarily single source to work with the failing second source to meet market demand.
I'm afraid that way some know-hows would ineviably leak from A to B which is unacceptable. Though protected by patents etc. themselves, they would allow A's rival gain valuable insights. So, no way this might happen.