What are your limits for your partner's friendships?

I am somewhat dealing with a situation where one friend (female) wants a divorce and another friend (married male) is spending hours a day with this person. They consider themselves best friends, although I think their friendship is relatively new (within the past year?). They do both hang out as couples, but I think this friendship is picking up the marital slack, in terms of emotional connection and helping with tasks around the house.

In my opinion, it is completely inappropriate.

It make me think about what my limits or boundaries are for my partner. In what contexts are his friendships OK or not OK? Is is based on time? Intimacy (non-sexual, I mean)? Being alone? Complimenting each other’s physical appearance?

At what point does a friendship take away from a marriage? At what point could a friendship become an emotional affair?