Hi Jennifer,
I’m sorry to hear about your friend’s loss. Unfortunately, I don’t think resin would be a good choice to preserve his boots. Resin hates moisture. While I’m sure his boots aren’t wet, they still have some moisture in them. They may look good in the resin right away, but over a period of months, they may begin to develop dark spots.
My best suggestion would be to search for someone who does historical preservations of clothing and such.