How friendly is “too friendly”?

My husband works in a female dominated industry so he is surrounded by women all day long. However he is only really friendly with one coworker. They text each other about every 3 months and they have caught up twice for coffee, just the two of them. She is attractive and about 6 years younger than him.

I’m not sure if I’m being paranoid/insecure, so what would you personally feel is inappropriate when it comes to work colleagues?