Delta recommended the following for a corroded cap assembly:

To remove a corroded Cap Assembly from a single handle bath valve, wrap a cloth around the joint and saturate it with boiling white vinegar. It might take more than one application.

Personal Experience:

If the above fails, unscrew the plastic ball retainer all the way out, and remove the ball. Apply vinegar to the inside of the Cap Assembly or spray in PB Blaster. If this fails, heat the cap with a propane torch.

Current Problem:

I want to replace the seats in a faucet for a friend but, so far, I haven't been able to remove the cap assembly or to remove the plastic ball retainer. She has been applying vinegar to the cap assembly joint as well as to the ball retainer area joint but, if I still can't remove either, what else might work?

I want to avoid replacing the faucet because it would be very inconvenient and time consuming in this situation.

A pipe wrench will work if nothing else does, but you will need a new cap. At that point, consider a new faucet.

The problem using a pipe wrench is that the torque applied by the wrench is resisted by the copper feed tubes inside the fitting. And, they're easy to damage by twisting the cap nut too much. If damaged, it's time for a new faucet as they're not repairable.

Bob.