psychology is simple
its the thought of if the friend is banging her ex now. then her friend might/must have wanted to before the divorce and so thoughts of betrayal that the friend might/must have cheated with him whilst first marriage was going on. or that the female friend may have been less loyal to alice in the first place and just wanted some frank meat between her legs all along. pretending to be friend with alice just to get reasons to be near frank

yes it might not be the case. but thats not the point its what alice might be thinking could have happened

the other aspect is things like. alice the now ex wife cant have open conversations with mary about the divorce and complications of the first marriage because mary is now getting pounded by the very guy alice has negative thoughts about. so from a therapy stand point. alice cant openly vent her frustrations about frank to the woman now yearning for franks genitals. so its a loss of trust and companionship

its just basic psychology. no one wants to talk to a person knowing what they may say might get back to the other party they are talking about. no one likes to open up about someone where the person you are talking to might be biased towards the person your talking about.

its a trust and loyalty thing.