Hey Madcap
You seem to be a really nice guy. First of all, let me say I don't think guys need to put women on a pedestal; you don't need to treat your girlfriend like a queen and serve her, just treat her like an equal human being.
And I can understand your suspicion. I'd be suspicious too if I was you.
I would confront her with your observations, even if you two have already talked about it befire.
It feels like she has continually lost appreciation for you based on your post here, I would tell her that too. Just see how she reacts and what she says.
If she really lost interest in you, I think it is better to make a conscious decision to split up than silently waiting for her to fade away from you, while your self esteem is shrinking all along.
But maybe it's not even that bad. Maybe her dressing up is just ment to show appreciation for a friend she doesn't see so often; in a non-sexual way...
If she visits him again you could make a little surprise-visit... if they're doing maths you can just shrug and say you felt like dropping by or bring some food from subway or so. If they don't open the door or behave like they've been trapped or uncomfortable.. That could be a sign.
Just collecting some ideas, hope it was helpful...