Its a 1/10 gulden from the Netherlands Antilles. If the date is 1954 as it looks to be, this would be the first year of issue. 0.6400 silver.
Nederlandse Antillen 1/10 gulden 1954 edit: dang .. beat me to it haha. I just google imaged "1/10G coin" and it came up as about the 60th result ahha.
15 mm is about half an inch... perhaps he didn't have the proper measuring tools to measure that small of a coin? idk.. Maybe he is buying it off ebay.. who knows.
Nope, orange is right i didnt have the thing to measure it, or the patience for that matter , no- i just found it in my collection lol o.o