Hmmm. I would just tell my MIL that the papaya and chicken combination doesn't go together, taste-wise. That the combo of this particular fruit and chicken is weird, and it's not very successful. Maybe other people like it, but it's not a good combination.
And I'd gush about how nice she was to make it (and give her a hug). And be sure to linger in my conversations with her.
But I wouldn't go the "doesn't agree w/ my stomach" route. I'd just tell her, the food combo didn't work, at least for me.