Is telling my mom's longtime friends her diagnosis violating her HIPAA rights?

I think you could contact those friends and let them know you are concerned about Mom as she is lonely and your afraid depressed without actually giving a diagnosis. You can let them know not to take it personally if they don’t hear back from her too as she seems to be having a harder time doing those things these days even when you know hearing from dear friends or family lifts her spirits. You don’t have to discuss any details of her health or diagnosis, if they ask tell them they will have to ask her that and can still let them know she needs them. You need them. You are really just asking them to help you help her because you are concerned. As a side note if you have POA I think you are cleared legally anyway and you aren’t acting as a healthcare professional in this case you are acting as a family member and can legally share whatever you know I think.

The other thing you could do is in the proper moment, maybe after she hears from one of these friends, ask her who she would want contacted should anything happen that lands her in the hospital. That should give you a good idea of who it’s ok to share at least the things you deem appropriate with.