I believe every relationship we have should point us to Christ! Every relationship should cause us to grow more like Him.
I've warned Christian teens dating non-Christians: if this relationship doesn't point you closer to Christ, it will only point you farther away.
Do I believe you can't be friends with people who don't have the same faith as you? Of course not! I believe in those friendship we can be challenged to grow in our walk by being an example of Christ's love, grace, and truth to them.
I say again, "be alert!"