I must agree, I played a holy and disc priest up to 85 and then al the way up to when patch 4.3 was released, and it's hard when you first ding 85. As Rilgamon said, regeneration is a bad joke when you've just dinged and haven't got your hands on some shiny loot yet. And it's not just for priests, all healing classes experience this
So keep on fighting, as she gets gear she'll notice it gets easier!
As for addons, I was using Grid as a healer. It's very customizable and I love the corner indicators. It's like small icons on players that you currently have renew, prayer of mendin and shield on
Since I haven't tried any other healing addons, Grid2 is the one i'd recommend