However, take a can of this, add a jar of that, and top it with a can of these is not really cooking despite warming it up.
No it's not cooking. And not the way I would do it. What I don't understand is why it bothers you (and others here) so much when other people do it.
Case in point: My host, where we spent Thanksgiving, made a couple of casseroles as sides; one hash browns, the other broccoli. In both cases, pre-shredded cheese and cans of undiluted cream of X soup were among the ingredients. Both the broccoli and hash browns were frozen bags. Basically, the sort of thing you're talking about. Is that cooking? Not hardly. At best it's assembling to order.
But.....not only did it not bother me, I enjoyed portions of both.
I could also make the point that your friend is at least making an effort to produce something. I belong to a club that periodically has pot-luck dinners. One couple can be depended on to bring a bucket of KFC as their contribution. Personally, I'd rather have your girlfriend's dip. But not my business either way.
Aren't we talking about pet peeves?
I reckon so. And one of mine is people sneering at other people's food preparation methods.