I have never heard of hot water in a bong making the hit smoother i will have to give that a go next time, but with regards to the thread if you're not willing to spend a chunk on a Vap which means you would rule out a good an expensive bong, have you tried putting ice in the bong it cools the smoke and makes it much smoother, also It took me a while to get decent weed that wasn't harsh, so might just be a case of trying a different source mate.

One last thing is that she will get used to it my advice is to keep taking the hits and she should get used to it remarkably quickly.