No matter whether a coin was dug or not dug, any verdigris needs to be arrested (stopped) to prevent any futher damage to coin. Once the verdigris has been arrested as best as possible, then a soak in Extra Virgin Olive Oil will stabilize the coin, hopefully from further damage from corrosion. Extra Virgin Olive Oil will not hurt a Copper coin but it may darken the coin somewhat. Extra Virgin Olive Oil is used by many Coin Dealers and myself to loosen dirt, grime, corrosion and other crud on Copper coins and to stabilize them. If a coin does not have dirt, grime, corrosion and other crud on them, then a light soak in it will stabilize the metal and hold off any future corrosion damage. If it does have these, the coin may need to be soaked in it for several weeks up to several months. When removing the loosened dirt, grime, corrosion and other crud, I always use a green Rose Thorn as it will not scratch or otherwise damage the surfaces of Copper coins. This was taught to me by a longtime Coin Dealer. I know that he has submitted many coins for certification and grading to quite a few of the Third Party Grading Services which he soaked in Extra Virgin Olive Oil, removed dirt...grime and other crud from them, patted dry, then 2x2'd before being submitted and he never had a problem. I do not know the specifics of each coin or who certified and graded each but do know, that the coins did not have verdigris on them. They were just dirty and some even yucky before the went into the Olive Oil but after the soak and dirt, grime and other crud was removed with a green Rose Thorn, they looked great. Just remember, to never submit a coin that has verdigris on it for certification or grading unless you use a Conservation Service that they provide before the coin is certified and graded but even then, the outcome may not be to your liking. As a general rule, it is generally best to sell most coins in the raw that has verdigris on it instead of having it certified and graded as the grade and notations on the label of the slab can be detrimental to the sell of the coin and the price it fecthes. The exception to this rule, would be extremely rare coins where you want to show that the coin is genuine.
Frank
if they can get 55-70 with environmental damage, then that should be a better ballpark estimate
I would have taken that offer since its 100% - 150% higher than what the coin is worth... 
