You need to have them appraised by a numismatist, they need to be graded to determine the value. Dont allow someone to give you a bullion price unless they are completely worn and possibly unidentifiable, ie: no date!
I would suggest to have them judged by a couple of places, unfortunately there are unscrupulous dealers that will tell you a coin is worthless, when in fact it is worth a great deal.
One example is Indian head nickles, a good one can be worth thousands of dollars! Some are 30K or so.
If you have several excellent examples, ie: mint condition, or uncirculated coins, I would suggest to have them "Slabbed" It will be in a sealed container and graded, most dealers will accept, the grading and pay accordingly.
PCGS is a reputable company, also NGC. You can contact the ANA in Colorado Springs for a reputable dealer in your hometown, at 800-367-9723 or www.MONEY.ORG
I would recommend that your friend educate himself as best he can and dont be in a hurry to sell, so he can get the best deal. He also may want to auction the valuable coin to make the most from it.
Numismatist is a magazine published by the ANA, should be able to find it in the library. There are many advertisers willing to handle the coins.
ps: at the time of the sale check the bullion value and compare to make sure you are getting a good deal. Right now Gold is about $897.00 an ounce, Silver is about $16.00 an ounce. So if you are offered less than bullion price ask why! Coins in good shape should be quite a bit more than bullion.
pps: A local coin club would be a good place to find a knowledgable person for advice of the value of coins, without cost involved. Most club members would be glad to look at the coins to point out valuable coins to have appraised. Also only handle coins by the edge and dont clean them!!! You can lose a fortune cleaning them, and it can be noticed if coins are cleaned, even rubbing with a soft cloth!!!