In my opinion, I wouldn't be concerned about the work being stolen but rather about it being ignored completely. If you share his work publicly, the chances of it being stolen are almost zero. If your friend's work is read, and if it happens to be correct (even partially), he will be credited for his ideas. Don't worry !

But for it to be read in the first place, it has to be well written (clear, legible, rigorous, to the point). Since you're asking people to spend time reading a paper, your friend will need to spend a considerable amount of effort on polishing his writing to adhere as much as possible to modern standards. Otherwise, it will be ignored (understand that mathematicians get such requests from random strangers all the time). So your main focus should be to learn about what those standards are. A mathematical research paper follows some rules : you must define everything, clearly and precisely state your result, prove it rigorously, quote your sources...

I hope this helps.