Unfortunately, I believe that no treatment for this frog would be best. UNLESS you see that it is suffering, or it was human caused (kids, dogs/cats, etc), it would be better to "let nature take its course"... as much as I hate to say it.
Amphibians seem to do well will traumatic injury, it is likely that it may be able to survive just fine without the leg.
The only other thing to mention is that amphibian malformations (missing or extra legs) are well documented in the US, keep an eye out for other frogs with similar problems. In which case you may have a more serious problem on your hands.
Despite my recommendations, I sincerely appreciate your concern for the frog!
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"I know the human being and fish can coexist peacefully."
Governor George W. Bush, Jr.
"Sometimes I think the surest sign that intelligent life exists elsewhere in the universe is that none of it has tried to contact us."
Calvin and Hobbes (Scientific Progress Goes 'Boink', 1991)