I would want to be told, and have been in situations where I BEGGED to be told the truth, quite literally, and was still lied to. It is horrible for someone to pretend there is nothing going on, keeps taking me back or keeps talking to me, etc. If I am having trouble getting over someone, the last thing I need is for them to be kind just so they won't feel bad.
It may sound brutal to an extent, but personally, I would tell her. Whether or not you guys stay friends after that is up to you 2, but she does need to know now so that it doesn't hinder that. Would you really want to be friends with someone who didn't tell you honestly what was going on when it was going on? I wouldn't be able to find a ton of trust I don't think after that, and would probably take a lot of what they said very lightly. Is there a chance that if you put it off that she might find out for herself before you have a chance to tell her? Do you guys have mutual friends with whom she is still speaking with?
If so, it might come out that way, and somehow I think it would hurt a lot more to find out for yourself and feel betrayed to some degree rather than being told right out. It would possibly help her get over you as well.
If she's upset and sitting around thinking you are single still, then that generally means there might still be a shot at it. If she is upset and knows you are with someone else, she might go the other direction and just get really ticked off, or she might plummet further into depression, but I'd bet it'd be quicker for her to get over it if she just knew. Just be prepared, the emotional tie to telling her could be rough, but overall I think it would be better for her.
Just my opinion.
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Sorry for the long post...
I'm my own guinea pig.