YABVU!
I agree with you that McDonald's food is absolute trash and I am appalled by McDonald's practices on many fronts. I'd hazard a guess that my opinions on McDonalds are at least as anti-McDonalds as yours, if not much more - I've protested against them, I support the idea of fat people suing them, I took a keen interest in the McLibel trial and I talk about Fast Food Nation (great book, you should read it!) to anyone who'll listen.
I don't (and won't) buy DS fizzy drinks, crisps or highly salted food. I encourage him to help me cooking and I think it's very important that he learns to appreciate good food and has a healthy diet.
However if I was in your situation I'd think "yuk" to myself, but let DS enjoy it. (He'll know my views on Md's as soon as he's old enough to understand!) I wouldn't judge the other mum. Different people have different priorities. In fact, I would be grateful to her for looking after my son for an afternoon.
I accept that I cannot control everything that goes on out of the house. I also understand that if I limit DS in a way that singles him out from his friends (e.g. not allowing him to go to a McDonald's party) then this could actually do more harm than good in the long term. If you make it absolutely banned it makes it forbidden fruit, and what easier way to rebel ... ?
Lots of important things happen at children's parties - bonding between the children, creating shared memories, socialisation etc - even at McDonalds.
All I feel I can do is show him what a good diet is, and make sure he eats well at home (and school, later) and then the rest is up to him. If you are mentally strict you are shooting yourself (or more accurately your DC) in the foot I think.
Or - in a nutshell (sorry I'm in waffly mode today 
) It's a party FFS! Eating one McDonalds isn't going to unbalance his otherwise healthy diet. YABVU and your comment "Look you go to a party you kind go make an informal contract as to what will take place. Macdonalds was never part of the deal." does come across as really quite control-freakery (sorry!)
If you say something to the mum, you will come across as a mentalist! And (this is important) a very ungrateful one at that. This woman has entertained your son for the afternoon. If you're too much trouble, she might not invite him next time, which would be a real shame.
I know it can be hard but you need to relax and trust that the good lessons you teach him at home will stick in the long term, and accept that other people have other priorities. I would never take my DS to McDs for his birthday. But I wouldn't judge someone else who did (although I might hope for a good moment to explain to the mum that there's poo in processed meats - see Fast Food Nation if you don't believe me)