My take on it (I am not homophobic, but a realist) :

Straight men are interested in women as mates (sexual and otherwise). I suspect lesbians share this interest (at least more so than straight women). I personally know a few who see "conquering" married women as a real challenge...much as some men do. So, I would not totally rule out that she has such an interest, regardless of how remote success may seem, (beyond friendship). If so, it may not matter whether P-vine is solid with you or not....if she is interested in P-Vine, at the best it may just lead to more frustration for both...at some point. So, at some point this woman must spread her wings elsewhere..because I suspect there is no sense wasting such effort on P-Vine.

So, what could a sensible course be? First P-Vine should clearify the limits of her interest in the relationship and make it clear that she does not keep anything from you. And, I suggest you should discuss with P-Vine you being included in the friendship, to further reinforce that it is only friendship...and that you folks have a solid relationship. If she needs more personal advice...this would not limit P-Vine providing some personal advice. Who knows, it may end up in a great 3 way friendship, and you may also be able to offer some advice and friendly support.

But, that's just my take from a distance.