Yay, an opportunity for me to give back some advice
I used a NS for the first couple months with Colin after my nipples became very bruised and sore in the hospital. I kept using them after I had healed partly because I was afraid I would end up getting hurt again.
I have finally weaned (day 2 of no shield use at all!) but my LC wasn't concerned about long-term use,
as long as they're used properly. The key is to make sure that the baby is latched properly, even with the shield. Unfortunately, this was my undoing in the early weeks -- the shields I had were too big for my LO, and he was sucking on the shaft of the shield, and thus not stimulating my breast and not getting enough milk. Once I traded the ones I had (the standard sizes) to an XS size (he was less than 6 weeks at this time), and started latching him the same way I would with my bare nipple, compressing my breast down to a "nipple sandwich" and making sure he got my breast, not just my nipple, in his mouth, my supply came back and he started getting more milk.
As long as it's being used properly, my LC said there was no harm to long-term use because the silicone ones are so thin, they don't interfere with breast stimulation or milk flow. IMO, however, it is easy to use them incorrectly.
I personally weaned more out of convenience, but I'm keeping them around in case I get sore nipples or if he's just having a bad day and I want to make it easier for him. But TBH, I started feeling like the shields were more uncomfortable than having him just latch bare.
At first, he wouldn't take my bare nipple either, but again, I wasn't using them correctly for a long time and he didn't really know how to breastfeed properly. First I started making sure he latched properly with the shield, then I started offering my nipple the same way (sandwich!)
In the beginning I would start with the shield and then remove it after my milk got flowing, but honestly, I found I had better luck starting without the shield. I'd hand express to get some milk going, so he had an instant reward -- although at first he would just lick the expressed milk off my nipple. "This is a good deal -- Mom does all the work, I just drink it up!"
We went from him taking the bare nipple for a couple minutes each feed, to him eventually staying on longer and longer by himself, until now he isn't using it at all. In the beginning he'd bob on and off when he just had my bare nipple, but I started holding his head (gently!) against my breast so he couldn't pull off.
One tip: I put the shield back on the moment he got frustrated, if he looked like he was going to start crying, because I didn't want him to associate nursing with stress and have him go on a "nursing strike." But eventually he started using the shield less and less.
The tough part was getting him to open his mouth enough for a good latch because with the shield it was easy to just poke it in his mouth. But I just kept teasing his lip the way my LC and videos showed, and eventually he would open up and I'd stuff the nipple in.
I did a lot of googling when I was figuring out how to wean, and read a lot of personal accounts from women who used the shield for several months until one day their baby just "got it" and started nursing without it. My LC pointed out that once babies get older, their mouths get bigger and it becomes easier for them to BF, so in many cases it's a self-correcting issue.
But again, articles I'd read and my LC both said that as long as they're used properly there's nothing wrong with using them long-term. And of course, if using the shield keeps her BFing (like it did for me) when she may otherwise have weaned, then it's definitely a plus!
Personally if she has concerns about whether the baby's getting enough milk or if she's just sick of using the shield (the way I was), I'd suggest she see a LC to make sure she's using it correctly and the baby's still latching onto the breast and not just sucking on the shaft.
Here's an article that might put your (and her) mind at ease:
http://thetruthaboutbreastfeeding.com/category/research/nipple-shields/
