I had not heard of this product so I did a little Googling.
Here is the Bo-Nash website including videos:
http://www.bonash.com
Here is a PDF I found on how to use it:
http://bonash.com/bi.pdf
Nowhere in the above could I find any reference to it mending leather. How did your husband happen to purchase the product? Do you know for sure it has been used to repair leather? My concern would be the amount of heat needed for the bonding process; heat could damage the leather.
If I *were* going to use a bonding agent with heat to repair leather, I think it would be a lot easier to use a non-powdered fusible such as Steam-a-Seam with a cloth patch (same color as leather) providing reinforcement. Is there a hem area where you could test if (1) iron heat would discolor the leather and (2) whether a fusible would actually hold on leather?
I went to Amazon and found this leather repair kit with reviews:
http://www.amazon.com/Leather-Repair...dp/B0002Q6362/ . This type of kit does not seem to require heat, so might be a better way to repair the tear.