This would be my solution as well.
But I’m curious… why were you poking around between the cheese and the lettuce in the first place? Do you make a point of dissecting any sandwiches offered to you in the presence of your host? Or were you specifically checking for boogers given the state of the apartment?
Normally I’d never notice, but it was haphazardly assembled and I saw just one… but, I suspect, there might have been more. When I saw it, I said I had a stomach ache and she proceeded to eat it.
And you thought those were sprinkles on your ice cream.
You’re using ‘‘dried boogers’’ as a metaphor for something else, right?
Ants? Cat hair? Please tell me yes.
Booger Nights, perhaps?
I’m afraid not.
Well, duh! It doesn’t start out dried.
I wonder why.
This is not normal behavior. Putting bodily waste on furniture or food is not a sign of a healthy personality. Please help her.
But how can I help? We’ve only known each other for a short time… wouldn’t it be inappropriate for me to question how she lives her life? 
I like her, I just don’t know what I would accomplish by saying anything (besides possibly a hateful glare) and I don’t even know what to say.
Yes, this is the type of thing where you’re not doing her a favor by keeping politely silent. Part of friendship is having people who can be honest with you.
How about “This is an astonishing collection of boogers! You know this is kind of gross, right? Have you always done this or do you just really hate the furniture?”.
I hadn’t realized you’d known her only a short time. I had imagined she’d been a normal seeming coworker for some time and this just happened to be the first visit to her house. I would have run screaming about the boogers everywhere, let alone on the sandwich. :: shudders ::
I laughed aloud! Oh but how harsh!
I was hoping that this was one of Skald’s hypothetical situations.
Tell her. Explain that the reason that you’re telling her is that you want to hang out, but it’s a weird situation.
Or just deposit your own boogers.
Maybe make little booger people out of them and arrange them in interesting positions.
Shag her anyway. Then wash. Often.
You say,
“Oh, my! It looks like something’s gotten into this sandwich! Never you mind, it happens. I don’t mind making myself another if you don’t mind!”
With a smile. Then you gauge her reaction.
Haha, uhhh, I’m not interested in having sex with her.
I suppose that might work, if she ever offers me another one. 
You know what’s really bothering me about this scenario? That booger was deliberately placed. Carefully placed, between the cheese and lettuce. No way does it just fall in there, especially considering they were everywhere. Which means she was hoping you’d not notice it and eat it. Which means she wants you to eat her leavings. Is she casting a love spell on you? Did you wrong her somehow?
I don’t remember wronging her… and she seems fairly heterosexual (she regaled me with the tale of a man who she loved but never returned her affections).
However.
I have a theory… I’m not sure that she places them on surfaces but instead, they just fall out of her nose.