a little background; i am in my first poly relationship. my Mistress, 52F, has been married to her husband, 50M, for 32 years. Her husband had two gf's until one inadvertantly outted the other to her family that didnt know. He now has only one gf. Mistress is my only relationship. i am going through a rough divorce right now. my ex cheated on me several times.
that said, i find cheating unforgivable, based on the reasoning that if you cheat once, you will cheat again. your husbands gf, for whatever reason, has cheated, and in all probability will again. i believe you are right in your discomfort in this. letting her off so ealsiy may be detrimental in your relationship with your husband in the near and distant future.
my Mistress and Her Husband have what they call "Ultimate Veto Power" in reguards to their extra-marital relationships. any and all lovers they have are subject to the approval of their spouse first, and throughout the life of those relationships. before either starts a relationship, the other meets and makes a relationship with the other and decides if this is a beneficial relationship or a toxic threat. all parties agree to the veto power and that His and Her relationship comes first. this must be agreed on by all.
i tell you this in the hopes that you will step back and re-evaluate your relationship with your husband. you married him. he married you. if you love each other, and im sure you do, you need to value this relationship higher than your bf and his gf. they need to know that you do. that you're united in this relationship first. that all others come second. if anybody (cough cough his gf cough cough) senses that you are divided, they will exploit you and him and know that neither of you will take a stand. unity is a must.
i wish nothing but the best for you and all your relationships. please dont take this as insult.