Socially acceptable to email a connection of a connection requesting information?
Ideally the person who knows both of you would write a short introductory email and send it to both of you.  You could request that he do that.

If he doesn't do that, you can still mention that he suggested that you write to this person at the beginning of your email.  Just use a succinct, mildly friendly, and professional tone, and you'll be fine.