I've always heard if the baby is under 6 months, than relactating is usually successful. I haven't heard much research on older babies, but thats probably just because it hasn't been done (the research, not the relactating). I know when my sister was 18ish months, she'd been done nursing for several months. My family went on vacation and on the flight she asked to nurse, nursed through the whole vacation and built a supply back up for my mom, even though she'd been dried up. So I know it's possible, of course as soon as vacation was over and my sister was no longer stressed, she no longer wanted to nurse and my mom had this huge supply again!
I'd suggest pumping and getting her to attempt breastfeeding as often as possible to reistablish the supply.