You wrote this: But we don’t know what he has or hasn’t tried to do to fix it.
Yes we do. OP has laid it out. Precisely nothing. His interest in spending time with her is waning and she gets little or no response if she suggests doing some of the things they used to enjoy together.
In actual fact, it’s less than nothing - he now knows how upset and anxious his behaviour is making OP and when confronted he quite clearly said he has no intention of changing anything. ‘I can’t change my feelings, that’s just how it is’.
And yet you posted to the effect that maybe she had used these same words to her DH to explain a waning sex life. On what planet would you jump to that as a reasonable explanation when it’s actually some sick version of tit for tat ? What evidence is there in any of OP’s posts that they didn’t have a good marriage before this friendship developed ? Apart from anything else, it’s absolutely no justification for having an affair. If you can’t address what, if anything, is wrong in the marriage and leave if you can’t work it out, you’re not being honest with either yourself or your partner.
You’ve presented a mans’ argument - no, a mans’ excuse. Which is why I challenged you, as have a number of other posters. OP’s main concern here is not, or shouldn’t be, whether he and his ‘friend’ have already had sex, or even that they are planning to initiate a physical element. His response has indicated a deep and intimate connection that he knows deep down is inappropriate but is not prepared to give up, even if it puts his marriage on the line. That’s what OP should be focused on because that’s where the real infidelity lies.