I guess it depends: If it's a work colleague - for example, they have no choice but to work together/get along well, that's one thing. Seeing each other outside of work with other colleagues present is still kinda ok. But the 'grey area' starts if they're seeing each other alone for drinks/meals/nights out etc. I'd have 'a problem' with that if I were in your shoes.

Anything that's not comfortable in your marriage should be open for honest discussion. If their friendship is making you anxious and worried-he needs to listen to your concerns. Maybe he'll reassure you. Maybe he'll tell you it's none of your business. You won't know until you raise the topic, and his response may help you gain insight into the situation.

If he's texting her in the evenings and weekends, I'd be suspicious that either he, or she, is looking for more than just 'friendship' potentially however. Unless they are texting about a shared hobby/sports team etc where 'banter' is the norm.

Some men do have female friends, but this ok only if your partner is open and honest about the friendship. If it becomes secretive or he prioritises being with her, over being with you-that would be a 'red flag' for me...and I'd stand my ground with an ultimatum.