I can understand why you feel usurped OP but there are some points of your post - and those of subsequent posters - that I'm very uncomfortable with.
You feel insecure because she's a bit younger and attractive and your husband likes her as a friend. She's not making any kind of play for your husband that you can see (or you would have posted it). Your husband is being upfront about his contact with her. Is he looking to find another job in a different area of work?
You won't find this very comforting but people in a marriage can and do fancy other people. They just (hopefully) acknowledge this to themselves and safeguard anything happening, shrugging it off as 'one of those things' because it is. Workplaces are prone and opportune for meeting new people but not all of it is sinister. The lunch meetings, how many have there been and with what gaps in between? The dinner meeting, was it just him and her or both of them with other colleagues?
How often does your husband take you for dinner - or look after his child(ren) so that you can pursue interests of your own? Has he done anything that makes you think that he fancies this woman? Is he less attentive to you?
It's your husband who needs to make the changes to his relationship with her so that you're comfortable, it's not this woman's job and she has done nothing wrong. I can feel the hackles rising in this thread and I don't like it, not one bit.
The other person who needs to step it up is you, OP. If your husband is not giving you cause for concern and worry then your jealousy and anguish over this woman is misplaced and you really need to work on your own self-esteem for your own benefit.
This woman, if she has done nothing more detrimental than be younger and attractive, is not your enemy or any other rotten and much-vaunted suggestion. It's a shame that women feel so badly about other women who have done nothing to deserve mistrust and suspicion. Some women are just horrible, save your ire for them.