[QUOTE=Arvin b;6455620]when we say one of my friends' girlfriend it seems that your friend has more than one girlfriend and you are talking about one of them.]
well,
yr sentence starts with "one of my friends" up to here, the word 'friend' has to be plural and as far as I remember when the noun is plural the apostrophe has to come after the plural "s" and it has nothing to do with girlfriend, besides the word 'girlfriend' is not plural
so I don't agree with you
.on the other hand "mine" is a possessive pronoun and maybe it should not be used with an apostrophe
exactly
well this sentence can be said in different ways:
one of my
friends' girlfriend
my
friend's girlfriend ( to shorten it we can omit 'one of')i
the girlfriend of a friends of mine (I think this one is grammatically correct , but not a good one, we've made it complicated, no need to do that)i