Yes, you can use Farmer's reducer on silver-based negatives, but be careful: You must not leave the negative in the reducer until it looks right, because the reducer continues to work a while when the negative is already in the water bath again. I would recommend not using too strong a solution.(posted 9172 days ago)If the negative is unique, consider duplicating it. Also, practice on a scrap negative.
Proceed as follows:
1) Presoak the negative in water. 2) Dip it into the reducer for a short while, then put it in water again. 3) Wait for a minute or so. 4) Repeat 2) and 3) until the negative looks right.
There is one more thing to consider: The reducer will also reduce the thin areas. If the negative is only difficult to print because exposure times are long, but it is otherwise OK, I would recommend living with the long exposures and not reducing the negative.