@Orangerose91,
Just because someone is attractive doesn't mean that they're available, interested or willing. Good Lord. Would you worry if his study mate was a guy?
She could be with someone, married, just got out of a **** marriage - you know nothing yet you are letting your fears and insecurities flourish. Maybe she is a threat; maybe not. But why start off this way? Wait until something happens before you start worrying.
Maybe meet her - ask your fiancé to set up a coffee date. You may be able to set your mind at rest.