I started really getting into vaping all because a person that works at the video game store where I hang out gave me a Ego-twist set up (Without me even asking, just as a Christmas gift!

). I was, and am still eternally grateful. That was over a year ago, since then I have moved onto mech mods and RDA's and I'm totally happy with where I am at. Since then I have had a few opportunity's to pay it forward in the same way that guy did for me and just like you are doing.
Luckily (In regards to vape stuff) I haven't encountered what you are dealing with. But sadly with other items/things I have ran into the same problem. I have noticed that not everyone is like this, it just seems that some people are more inclined to "take advantage" of situations. From my perspective I don't think people that do this are doing it because they are trying to exploit you. However it seems some people are just unaware when they might be crossing a line.
Because of this, try to go easy on this person. I like to (kindly) push people in a "self-dependent" direction. In your case, refer her to some websites or a local shop. This might not work, as it does not always work for me. If that ends up being the case you are better of jut being upfront with this person in a nice way. I have made the mistake of not doing that right away and it has cost me. Also, as I said before, this person is probably not trying to be overbearing so they shouldn't end up being offended if you are up-front with them.
If there's any point I want to make here its this. If you chose not to be upfront with this person and continue providing them with what they need they will keep coming back until you do something about it. Just because you wont pay for everything they need does not make you a bad person. Some people just need a little "heads up" about boundaries.
I would also urge you to not allow this situation to turn you off of "paying it forward". Without people like yourself and that guy at the game shop chances are I would have never gotten into this hobby and would still be smoking.