I don't believe Indians are allowed to take vehicles across the border, but I could be wrong here.
Get an official paper in English in which your friend authorises you to drive his vehicle. Probably best to get this notarised, but I don't know what the Iranians or Turks would expect.
The question is: how much risk are you willing to take to have your bike confiscated at a border, knowing that you will never get it back?
To me, the easier and safer option would be to sell the bike and buy another in Nepal, from where you know you can ride it with a carnet to wherever you please.
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