Don't judge, the child/parent relationship is pure and strong. first of all if he's well adjusted and not tired in the morning it's not affecting him. For her, she has the bonding and comfort from her child, if she wanted it different she would have it different. I think it sounds like it works for both of them. It seems it's just you it's affecting, so best sleep in your own bed and stay out of it...
my boys never slept with me on a regular basis, but if they where sick, or frightened from a movie or wanting to talk about a problem at school etc I would let them, never made a big deal out of it, funny thing once they all hit puberty they stopped wanting to do this. Theres so many problems with rasing kids ya gotta pick your battles especially when you have 3 like me, dont sweat the small stuff.
It is not uncommon for children to sleep with their parents and in some parts of Europe they have Family beds, where all the Family pile in together, however in our culture is is believed that children do better and have more dependancy if they have their own bed and room, I doubt if this mother has any inappropiate feelings towards her son, they are simply very bonded,also if there had been abuse of some sort with his Father that would make them want to be with each other more,from feelings of protection, Have you asked her when her son is going to stay in his own room and talked to her about how you feel. IHow about you do more with her son, play games and get him interested in doing some one to one things, win him over, and if you are serious about his mom and you then time will help.
It's not- no matter how you slice it - It will very probably only end up looking like you want in bed with the woman.
And there are still people in this world that think "your staying over for adult time" seems highly inappropriate.....