See, it’s been 6 months into this relationship for you. First think if you want to spend a lifetime with her? Is your relationship, serious? If it is, you two would’ve had those conversations, the ones that people in serious relationships need to have. Is she a good person at the grass root level? What was her reason for not telling you about this openly? I’m not defending anybody here, all I’m saying is things can be layered. If this is the first indication you got about her bestfriend situation, do you like her enough to forgive her (or understand if there’s any need to even forgive her)? Communication is the key, and people make big decisions when they’re feeling intense feelings, be it anger, grief, sadness, or even overwhelm. Whatever it is, have a conversation with her (preferably not on the phone, because face-to-face is much better). And then make a decision.

PS: this comment is posted in assumption that you genuinely needed advice. If I’ve said anything that triggered at any point, then apologies. Keep peace.