I have a question that may seem a bit childish. You say her relationship with him is long distance and she rarely sees him? Ok. So, if she's afraid to tell him about her new relationship with you, why not just use the distance as an excuse to end the relationship she has with him? It's definitely a cop out, and I will admit to that. But, at least for a little while until she can be totally honest, it's a way to make a clean break and possibly keep the friendship and avoid him treating her badly. You really can't argue with "I can't handle only seeing you only every other month and I'm growing closer to V and she's HERE". Not unless he says he'll pick up and move.
I'll sit back and wait to be bashed about my lack of honesty now.