Your wife’s BFF is in serious marital dificulty. Change in sexual orientation can be a “cause” or an “effect.” It may be that the marital difficulties have emerged because she is finally acknowledging her lesbian sexuality. Or she may be trying out a lesbian identity as a result of being so dissatisfied with her husband and marriage. Either way, your wife is not helping matters by “playing along” in order to get along with her old friend if that is what she’s doing. It’s understandable that the BFF would think your wife is also willing to explore a gay relationship since she isn’t drawing boundaries on phone calls, messages, and endearments.
Since they are old friends, a heart to heart talk between the two is in order. Your wife can be supportive without sending mixed signals. She can love her friend without letting her think she is in love with her. She can’t let her BFF think that the way out of her marriage is to hop into a relationship with her. If she wants to leave the marriage, the BFF needs to do so on its merits. She’s in no way ready for a new relationship since she hasn’t dealt with a separation or spent the time and attention necessary to heal from a failed relationship or to come to terms with her own sexual identity. If your wife really doesn’t want her BFF’s attentions, she should put the brakes on now. I worry that BFF is getting obsessive. This could be potentially dangerous if your wife rejects her in the future.
I do sympathize with your wife’s desire to support her friend. When we’ve been friends for a long, long time, we want to really be there during the bad times as well as the good. But I do think she needs to find a more straightforward way to tell her BFF that as much as she loves her, she’s not interested in being in a romance with her and that she is committed to you.
I wish everyone in this difficult situation well.