Need advice re woman at his work

[lillypilly7]

Hi I'm new here. I've been reading the posts and enjoying the advice and shared confidences. I hope someone can help with mine (. Here goes. Sorry its a bit long.

The background:
My fiance has a woman at work he is more than friendly with. At work she treats him like a partner and it seems vice versa, he takes her to lunch (and then pretends it was his boss - who is his best friend - who invited the 2 of them) and they share confidences and secrets. I'm sure he shares his relationship problems with her. At social functions she (and probably he) always puts his seat next to hers. She does the doe-eyed thing standing 2 inches from his chin when I'm standing next to him..They plan bbq menus for work functions (they have a lot of 'team' social functions) and then he brings home her salad platters to wash in our kitchen.Photos of our new block of land downloaded from his phone to computer accidentally also downloaded smilinggg close up face photos of her at the pub. I didn't even know he'd been there. His 'recently opened documents were all work-related except for a photo of ...u guessed it...her! I deleted it and didn't say anything - he has many copies anyway on other puters and drives.etc U get the picture?

He 'explained' all of this away...gets angry and looks totally furious and frustrated, says 'why am I here then?'. It all makes me feel sick. He still does all of these things with her and just avoids telling me, hides receipts (for lunches and drinks), emails, his phone etc etc. He is in charge of the ten pin bowling work function, he had to arrange the teams, I asked who was in his team, he 'couldnt remember (oh yeah?). I am very hurt by her continued presence in his day and his lack of consideration of my feelings.

We were supposed to go on a free houseboat weekend with her and her partner (yes she is married but her husband is very insipid and odd, and they seem very distant from each other) - I said I could stomach an evening event but not being stuck a whole weekend. He hit the roof and was going to leave me again. We ended up being interstate and so couldnt go anyway, thank goodness.

The current situation: My fiance now has a 25 years of employment special dinner,he is now a member of the honored Quarter Century Club at work. He has to invite 10 people to a free dinner, which must include his boss. He can choose the others. He only works in a small team of around 5 including herrrr plus some lesser familiar workers at other venues.

Should I put my foot down and tell him not to invite her? Should I say you can invite her but the engagement is off? Should I say nothing (the pain would be unbearable). Either way the whole thing is doomed. There will be a huge scene and he will leave me. I am miserable. I don't want to spoil his dinner but I want him to show me that I am more important to him than her and this is my chance to find out for sure.

I will probably get kicked off for such a long post but I am desperate for advice.

Fingers crossed,

Jill.