V, if Mash knows the woman, knows that she is still desperate not to lose the guy, even though they have been apart for 18m, it's just not cricket to throw all caution to the wind.
I'd put myself in the W position and I'd not be able to just dive right in, not while there is the merest whiff of impropriety. I'd want to make damned sure that everything was over and clean before I got involved. If she'd have booted him out, and wanted nothing to do with him, it may be a different story.
But the guy rocked up unannounced, on the look out, for sure. I'd want to see him walk the walk too before I potentially set myself for that potential grief.
There is every possibility that this guy could be genuine, that he may come right and it'll all be OK. He might just be marking his place, making his position clear, so as not to lose her again. He certainly seems to have feelings for Mash, so if it's all goign to be OK in the end, the big Happy Ending? Why the rush?
Mash, it may not seem too great an idea for you to hear this, but 6m is a very short time, if this guy is for real, is a serious possibility, I think that you could really benefit from spending more time getting over your XH and getting to know yourself, learning to love and nurture yourself, before letting someone else in.
Enjoy the ego boost, you seriously deserve it, but work on yourself and love yourself, as only when you do, will someone else be able to.