I wouldn't worry about it too much, i know my SIL still gave her boy a bottle around that time - I gave up the night bottle at around 6-7 months, but only because we would have to wake him up for it, and he would be awfully cranky.
I say that she will let you know when she is ready to drop it. Mine still wakes up now during the night, but settles with just a dummy. We only give him water if he has a temp or something which wakes him up, something to make sure he isn't getting dehydrated.
Not much help I know, but good luck!! Just follow your instincts.. you're doin a great job