Although I voted 'Good Idea' I do see many pitfalls and drawbacks.
First, there's no real chat functionality in VNC, so you'd have to make-do with notepad.
Second, you'd have to allow shared input so the other party can block your actions if they deem fit (always fun, fighting for control of the mouse) and at least to be able to show you where he/she has a problem.
And even then, a certified email pre-send, giving me total absolvement of any possible mishap AAR of whatever possible cause...
But I fear, agreeing with CoolBeans here, the matter is so complex that you can indeed end up spent a lot of time 'helping out'.
BTW, there's also the security part, firewall rules need to be set up allowing only
the helper access. Ruling out any
possibility for others than the helper to connect. And believe me, you're a target the moment you go online...
as it so happens I experienced it again myself this afternoon. The darn webport was only open to the outside for bout 10 minutes, and still some nitwit managed to fire about a dozen different malformed IIS targeted exploit requests at it (Learned a couple of new ones myself [