our bull started catching feelings... how did you actually end it without it getting ugly

M41 / F39, hotwife dynamic 4 yrs, we're in the unfortunate phase of having to wind down our 18 month regular bull because he is in feelings territory. it didnt happen overnight. it crept up. first the late night texts to her not about logistics. then small gifts. then asking her to lunch (no play) once a month. she enjoyed the attention obviously, who wouldnt, but she also flagged it to me about 3 months ago that something was shifting. last week he sent her a text basically saying he had real feelings for her and was hoping we could "talk about an evolution". very polite, not pushy, but its now on the table and it cant be on the table. we are not opening up our marriage in that direction, our boundaries were always clear that he was the third not a partner. now we need to actually end it. and i dont know the etiquette. is it her job or my job to send the message. do you do it via text or call. is there a "see you in 6 months once feelings cool" option or is that a delusion. has anyone managed to keep the friendship after ending the play. and the big one, how do we make sure he doesnt feel like 18 months of his life was a waste, because it wasnt for any of us, but its ending in a way that could feel that way to him if were not careful