Again, the issue wouldn't necessarily be Hezbollah itself turning on Iran, but rather individuals within Hezbollah doing so once they had a nuclear weapon. There would be a lot of incentive for an individual or subgroup to try to use the nuke for their own ulterior motives. For one thing, the United States and Russia offer huge rewards for loose nuclear material, so getting a hold of some of the components just to turn it over could very well be worth it for someone.
Even after that, I'm inclined to think if Iran were dumb enough to give Hezbollah a nuclear weapon that Hezbollah would be very hesitant about using it as described (on Tel-Aviv) as many of the people they claim to represent would be in the potential fallout zone and it would be very damaging to their credibility as a legitimate political party. They might not directly turn on Iran, but they might not use it in the intended way either. They might dissemble it to make several dirty bombs, which would be much more suitable for their goals but also would be much more traceable using nuclear forensics, or they could even attempt to reverse engineer the design in order to sell it to other terrorist organizations (some of which Iran might not like- regardless no country wants their nuclear secrets out on the black market).
Now what I suppose could happen is that an official in the Iranian nuclear chain of command could sell or give a nuclear weapon to Hezbollah or another organization without the support of Tehran. It would still probably be difficult for this to be accomplished successfully, but it could happen. Then again, this is basically the biggest concern about the Pakistani and Russian arsenals as well, so it's something we are already worrying about.
I just don't see the Iranian government knowingly giving a nuke to a non-state actor. Sure they might not get caught, but there is an awful lot chances for them to get caught and if they did they would be absolutely screwed. Not only would this basically guarantee them getting the bejeezus bombed out of them, but it would probably be enough for China and Russia to end any of their tacit support for the regime as well as they certainly fear the terrorists organizations in their own country acquiring loose nuclear material as much as anyone else.