When we went onto just cows milk, my LO's poos went crazy- 6-7 a day and always sloppy and huge and awful. Turns out his little body just didn't like that much dairy. As soon as we went to lactose free milk, it went back to 2-3 poos a day, much better consistency. So maybe your niece has a similar issue?

As for how to bring it up, hm, that's harder! I guess I would go in with the sympathy angle-
"gee, you must get so fed up with changing that many pooey nappies a day..." etc.

Good luck!!