I'm sorry you're going through this.
Part of the reason why affairs are so damaging is not just the fact that one partner has left for another person - it's the months, sometimes years of lying and gaslighting that goes with it.
These fuckers will lie for along time because before they leap, they want to ensure that they have a solid, soft landing, and this at your expense.
He doesn't have to be friends like this with this woman OP. That's his choice, and by disregarding your feelings in the way he's doing, it's clear he's going to prioritise that in front of your feelings and emotional security.
@LilySad91 Well probably not! It's bloody naive and gaslighting to encourage women to think of female friendships their husband is developing exactly the same way as they'd regard a male friendship, because opposite sex friendships are not the same. A lot men don't regard female friendships in the same way they regard male friendships, even if outwardly they would insist that they do (in certain company, so to avoid being challenged or arousing suspicion).
@JadeSquid The situation that the OP is going through now is often how affairs start - it happens right under the nose of their poor partner who must put up with being called jealous or crazy, until one day, they finally realise they were right all along and their partner was making a fucking fool of them the entire time.
So often, the woman in question isn't average looking Sharon from accounts, or menopausal Belinda that's a few years from retirement. It's not-so-coincidently someone attractive, and often younger. A lot of women who end up on here with these problems just know that something's off, and they have to witness, in slow-motion detail, how their husband is slipping away from them. It's a difficult grey area for them to manage, and just because things are hunky-dory in your relationship, doesn't mean the same applies to them.