IMO, your husband is giving you a line of crap, this is "not" just a friendship situation. Men don't really have these kinds of "friendships" with women, he is showing that very clearly in how they are both going beyond a regular "friendship" with adding in sexual
things about themselves, come hither is what that is all about.

He is also in "denial" with you too, so your concerns are worthy and he is being very dishonest with you. They are both being "dishonest" about this so called "friendship".
This is all about "selfish ego" in both him and this other woman.

And the sad part is that if they keep going and break up two marriages, and end up with each other exclusively, it will end up "burning out" and they may both end up once again cheating with yet someone else. Yup it's all about "selfish ego boosting".

They have a name for this set some years back, it's called the 7 year itch, which is right around that time frame in your marriage.

Time to put your foot down IMO.

OE