Would it make any real difference if the scenario was that I had this woman over to my house almost every time my W was on a (frequent) business trip, with W's knowledge and encouragement, for various reasons the idea of a romantic relationship was ridiculous, and we would only be continuing these obviously platonic dinners?
I'm just trying to look at it from your friend's point-of-view, and I think she's attempting to avoid any possible -appearance- of impropriety. The difference between now and before is (obviously) that your M is on the rocks. So you are potentially "available" to the opposite sex now whereas you weren't before. So even though you and she may have perfectly harmless intentions, you really don't know how your W would perceive it if you had her over. I am 99% sure that your W would suspect something no matter how much or little you might explain to the contrary, and your friend probably knows that.
