My friend has just boughts Cessna 150 and found out that the vaccum gyro horizon is not working properly. He opened it up and found out that inside of the instrument is covered up with a white powder. Looks like its a corrosion but I am not sure. If this is a corrosion why is it white? What cleaner sholud be used to get rid of this white stuff so that not damage the instrument?
Unless your friend holds mechanic certification with at least an airframe rating, he has no business touching that panel, removing an instrument, and most certainly opening the instrument. The instrument is junk, now. He can send it as a core to a repair station, but will likely pay a hefty sum to get it back.
Your friend isn't allowed to remove the instrument, open the instrument, or even open or separate the pitot-static system and attached lines behind the instruments. Your friend is operating illegally, and dangerously.
The work on the instrument needs to be done by the holder of a repair station certificate, which is certified to do that specific work. Beyond that, your friend needs to refrain from the misguided belief that he can or should work on his own airplane.