I would say something like "Friend and I have an agreement
that I don't tempt her with yummy things." I think that trying to eat better is common enough for people to understand, without any more details.
Please don't mention "yummy things" in the way that only sugary, hi-calorie, whip-cream covered in chocolate, divinely decadent (I digress, sorry) things are "yummy."
To a dieter, carrot sticks might be considered yummy. (Does that make sense at all?)
OP -- I think you're doing just fine and you're a great friend to include dieter in on some days when you bring in the healthier treats.
If someone should say something to you, I agree that a simple comment like "Friend and I have an agreement" should suffice, although I'm thinking that a bean-dip would be more appropriate.
I'm trying to figure out a situation why someone might mention something about your not offering friend a treat. Do you stand by your treat and officially serve individual portions asking each person "Would you like a piece?"
In my office, when someone brings in a treat, there's an e-mail or a general announcement "Treats in the break room! Help yourself!"
Just trying to understand . . .