My mother is trying to get back on her bike after years of knee problems + surgery, which so far has gone ok, but she has had some trouble with climbing hills. She's thinking that she should try to get some lower gearing on her bicycle. She currently has a "road triple" with a long cage derailer and a (relatively small, in my book) 11-26 tooth rear cassette.
I figured that with a long cage derailer, she should have plenty of flexibility in downgearing her cassette (with a new chain, of course). She took it to her local shop though, and they insisted that she couldn't downgear any more than to a 28 tooth cassette. When I called to ask about it, they said that even if she replaces her derailer, "with a triple it just won't work". I wasn't able to get more details, just a repeat response, even after I asked if the issue had anything to do with taking up chain slack or chain length on the large front + large back combo.
Every source I've found about the issue doesn't mention anything very triple dependent. Sheldon's site seems to think that shops are generally overly conservative on their downgearing recommendations, but that page seems to expect a bike with a double chainring.
What are the usual limiting factors on a downgearing conversion? What specific concerns should we have with a triple in front? How can I figure out how large a cassette I can get away with? And what ill effects can she expect if she violates the advice of the shop and puts a larger cassette on?