If she gave up her Polish citizenship then I think that's right, but if she never did then she can hold both. Also the OP writes that:
My mother is in the process of obtaining proof of her citizenship which is in the final phases
so it sounds like she's eligible for Polish citizenship/passport.
My parents naturalised as British but Mum got her Polish citizenship confirmed many years later as she had never given it up and this opened up the path for her kids to do the same.