Since you've been friends for a while, you probably are correct that she is down and lonely. She would also know that you would sense it, too, imo.

Rather than discuss the sadness, have you tried to just say, xxxx we both know you are feeling xxxx, and I want to be the friend to you I have been, what do you want me to do to help you? Say something like that in your own words, of course!

Maybe just acknowledging the feelings, she will open up when she is able?

I'm sorry she isn't validating your friendship right now, as you would want, but a real friend will stick around for a little while, imo.

Take care of yourself, you don't need to be angry. If she's as upset as you feel, then she needs everyone she can to support her.

You sound like a caring friend, imo.