they can help with pain of engorgement, but yes if they're over-used they can cause a drop in supply. it shouldn't be drastic though, best to just have her use her judgement. don't wear them all day and night, just when uncomfortable. i always loved to use the leaves straight out of the fridge when i was really engorged--feels great!
I was once told to only use them for about twenty minutes, every 3 hours, or as often as you feel pain I guess. I've also read that you could use them for up to several hours (I think it was in Womanly Art O.B.). Of course, if the supply drops, then eating a little oatmeal should help.