Sympathies, OP. People with dementia can be unbelievably stubborn about wearing incontinence pads - or anything else, for that matter. Of course, because they can never remember anything, they may genuinely think they don’t need them, but that doesn’t make it any easier to deal with.
People who have never had to live with this sort of issue can hardly ever understand what it’s like to have to cope with it.
I suppose you will have to put your foot down, say there will be no lunches at your house or anywhere else, unless you see with your own eyes that she has adequate incontinence protection on. If she gets into a strop, so be it. I know it’s all too easy for me to say, but I’d try to just calmly say, ‘All right, but if you won’t show me, no lunch.’
We had no end of dramas with my mother and showering/hair washing, when she really did smell, so I do know how very trying it can be.