I know there are other threads regarding
using UV light to remove the yellow/amber/brownish tint which many 1950's-1970's Thorium glass containing lens develop. I'm adding this one because I found a super easy to use UV blacklight flood lamp which operates at a cool enough temperature to allow the lens to sit right on the flood light glass! I've done two fairly severely yellowed Takumar 50mm F1.4 lenses and the results have been OUTSTANDING.
Lens #1 was a late 1960's vintage Super-Takumar 50mm f1.4 with splotchy dark yellow elements. After about ten hours on the lamp face down and another ten face up, the lens is as clear as the day it left Japan. Lens #2 was a Super-Multi-Coated Takumar 50mm f1.4 from around 1971. It was evenly yellowed. I did the same treatment to it with equally excellent results. Photos of its treatment and the lamp are attached.
The BEST part? The UV flood lamp is only $16.99 US on AMAZON. It is a 10 Watt output version. They also sell 20 Watt and higher versions, however those lights generate more heat and would require the lens to be moved away from the lamp to keep the lubricant from overheating.
All you have to do is set the 10 Watt HouLight UV Black Light Flood Lamp on a flat surface with the light lens facing straight up. Plug the light into a 3 prong 110-115V receptacle. Take your Thorium yellowed lens, open the aperture fully and set it face down, centered over the LEDs in the UV lamp. I put mine on a rubber collapsable lens hood to be sure the lens elects didn't contact the lamp glass. After 8-10 hours, turn the lens over so the rear element is now toward the UV lamp and let it go another 8-10 hours. Check the lens by looking at something white under good lighting. If there is no more yellow cast (coatings are a bit yellow, so allow for that) you are DONE. If there is still some yellow, give it more time on the lamp. Easy. The HouLight is rated for 5000 hours of use, so you can do your friends lenses Too.