M.G.
One of my son's is like this. He would go to my in-laws and they'd have candy and junk, and he's come home and puke every time. Birthday parties - he'd come home after - same thing.
I used to throw up every time I went on rides. I had a very sensitive tummy. Grew out of it.
My son is healthy otherwise. He is more sensitive to milk - we discovered this because he had a lot of gas. So we switched him off milk to lactose free and it helped.
Also he got sinusitis in the winter months as a kid, and we think the post nasal drip didn't help.
He's pretty much grown out of it now (young teen).
Added:
Ask MIL to not give her candy - and remind child what happens when she eats it. That's what we did. My MIL would forget because she doesn't like to say "no" so it was on me to remind my kid that he felt awful when he ate too much. So it's likely that he hasn't so much grown out of it, he just doesn't eat much.