Nope--not bad at all. Given that you can answer your own question, this is perfectly fine. I can see concerns about it being considered gaming the system--when you answer your own question it is upfront, but not when you answer a friends' question.
We need good questions+answers. Multiple times, a question is unclear and talking with the OP is one way to easily get what s/he means. In this case, answer the question, and clarify it if necessary.
Note that excessive "seeding" (cooking up questions) is sometimes frowned upon, and collaborating with a friend in this could be considered vote fraud.\
Basically, don't do this all the time. A few questions, that's OK. Not almost all of them. The tuition student thingy is one example where it can get too much.
Note that nobody's going to tell you (and I don't know) how the vote fraud mechanism works.
A relevant example of me doing this:
I once introduced a friend of mine to Physics.SE, and she posted quite a few things (of which I upvoted almost all--but that's because they came by me, in the course of exploring the recent activity--not because I targeted her in specific).
Anyway, there was one question she had. I myself had had this doubt a year ago (same reason as her, incidentally), and I'd solved it myself. So I posted my solution there. I don't think that's fraud.
Then there's my answer here. I realized that her answer was wrong, and I told her over the phone (similar to your case). We discussed, and she managed to improve her answer, as well as urging me to post my own. That's pretty OK as well--the post quality improved.
Basically, don't worry as long as you don't do it too much.