I totally understand. My Mom withdrew from her friends as well when the signs of dementia increased. My Dad helped hide it as well, and even though I stopped by constantly, and told everyone what was happening, people didn't want to believe it. Even siblings at one point -- it wasn't until they actually spent a day with her that they realized how bad it was getting.
You have done the right thing by moving her to a facility where she is safe and cared for. As I told another friend who is dealing with the same thing, you just do the best you can for the safety of your parents. I believe that your Mom's friends are probably feeling a little afraid that dementia/Alzheimer's could happen to them as well and that probably is the reasoning behind their silence. And, many people don't understand health issues until they have to face them. I don't know either, but sometimes you just need a good vent!