No, I suppose that physically, she could have told her friends.
But we all saw Xander's reaction in Entropy. We all saw how ashamed and guilty Buffy felt for having sex with a soulless vampire, for using that vampire solely for sex. Who can blame her, really, for not telling?
I justify her depression because I feel that her depression was justified.
Do I think she should have sat around and wallowed? No. And she didn't. She still accomplished her Slayer duties, she was "going through the motions" trying to be normal. She sought solace and comfort in Spike. Her friends, I feel, didn't pick up the slack and help her the way she needed to be helped. Spike didn't help her the way she needed to be helped. And that makes me feel horrible for Buffy.
The fact that there's a heaven would comfort you, I'd imagine, unless you were pulled from it. For the first time ever, Buffy felt safe and certain. She felt finished where she was, a feeling that none of us can really grasp. And then she was pulled out from that feeling, which must have been extremely traumatizing. She was tossed back into a world that now felt like hell compared to where she'd been, and she was expected, by her friends, to be as grateful and happy as they were. Her depression WAS completely justified. People have been more depressed for lesser events happening in their lives.