It can be really hard to see things clearly when you feel so insecure/jealous, and don't know what to do. I don't know that there's anything you can do. They work together, this is their job. Perhaps let your husband know he has a case of mentionitis and while you're sure she's lovely, it's not quite as riveting hearing all about her second hand as it is for him experiencing it first hand. I think your best best as long as he's behaving appropriately is to play it cool, try not to come off as insecure, and keep your eyes and ears open. While it's important to be honest with him about your feelings, unless he actually does anything wrong (for example meeting her in secret, meeting her after hours, texting all weekend) you've just gotta work on your own insecurity. If he wants to cheat, with her or anyone, he will. And no amount of worrying on your behalf will stop him.
For what it's worth, men and women can meet at work and become good friends. My general 'rule' about this is to involve partners quickly, so it becomes more of a foursome and is clear to everyone it's purely platonic. I met a guy through a voluntary job a decade ago and we hit it off despite me being a nineteen year old woman and him a 40 year old married man with a child. Just similar wavelengths, the job is very emotionally intense so you support one another a lot, and we coincidentally went through losing a parent at the same time.
His wife was and is super cool and treated me as a friend of the family, there's never been a sniff of anything but pure friendship the entire time and ten years later we have one of the most important friendships in my life, I'm essentially part of his family as an honorary auntie to his son and I'm doing everything I can to take the load off his wife as he is now terminally ill. I know she's glad of the support and not dealing with it alone.
Try keep an open mind, listen to your gut but remember it's not always accurate. I know sometimes jealousy can convince me something is inappropriate that my head knows really isn't. It's a powerful emotion.