Well, I have been lurking for probably a little over a month now, and you just made me register
haha. (won't ruin your question, I'll make a welcome thread in a moment.)
What I would do: Just go ahead and quietly empty her garbage. Don't make a fuss about it, if her trash can is by her knee just say "excuse me for a sec." and empty her garbage, just remember to bring a new bag so you don't have to ask her for one. If she looks at you weird, just say, "sorry, something in your sandwich was just getting to me." I wouldn't bother complaining to a higher up unless there is already a no food at the desk policy.
Downfalls?: she could think that anytime she doesn't feel like emptying her trash can she can toss something stinky in there and you will change it.
Most likely: she will say, "oh yeah, sure, sorry!" and if she recalls it next time she eats the same sandwich she might dispose of it differently. (but will most likely forget.)
Possible: she will become overly apologetic and snatch the bag from your hand do her trash herself and possibly yours, and after anytime she eats something she will ask you if it is bothering you.
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Det