な is generally masculine speech, but in this case you can't replace it with ね for neutral speech of the same meaning. いいね is just 'it's nice', while いいな implies envy or hope. If they can't say いいな, what would they say instead?
The one, which implies envy or hope is often written
いいなあ, or even
いいなあ～ and is absolutely fine for girls. I would almost say, that (especially in the envy-usage) is more often used by girls than guys, but this is maybe not a problem of speech, but more a problem of displaying envy via speech.
But there also is a manly
いいな, but it has a different meaning.