Anna, I'm sure that you will get plenty of worthwhile suggestion about your mom with the "stopped up" breast, so I just wanted to throw out the one thing that first struck me:
If this mom is producing fine with the other breast, why would there be any need to wean?  Indeed, there is little reason even to think that she would need to supplement; certainly not for more than a few days, during which she should nurse extra-often at the "good" breast, and perhaps pump, too.
I know a nurse, now an IBCLC, whose motivation for becoming involved with lactation on a professional basis was her own sad experience with her baby a few years ago.  She had one breast that the baby utterly refused to take, and after seeking counseling from a few sources, she reluctantly weaned.  A few months later she "discovered" that there was no reason that she couldn't nurse exclusively at one breast, if that was how it needed to be.  Heck, some cultures use only one breast; some women *have* only one breast...
Don't let misinformation cause this mom to wean solely because one breast "doesn't work" (although I suspect that the breast problem is fixable anyway, and am sure you will hear from others on that topic).
Sincerely, Willow 
Willow A. Ward, RN, IBCLC, in sunny warm Detroit, MI, where tomorrow it will probably snow again...