Dear Lucy
Marriage is an exclusive relationship in that we shouldn't be closer emotionally or physically to another person of the opposite sex, other than our husband or wife. However it should also be possible to trust each other. The danger is, of we spend a lot of time alone with someone who isn't our marriage partner that temptations may arise. It's very easy to say that nothing will happen to us, but many well meaning people have been caught out by this.
Perhaps your husband could think about why he wants to see this girl on his own and what he is looking for in the relationship. You could also talk through how you understand trust.
This article might help you to think this through.
Kate