Do not say anything to her. She will either tell him, resulting in him getting mad at you, or she will tell people at work who will then tease your husband, again resulting in him getting mad at you.
Texting has become very common. My boyfriend and I will text each other when we are at work because to talk on the phone isn't easy, and we might need to quickly communicate something to each other that can't wait until we get home or off work - say, if he's supposed to pick up the kids but has to work late, he needs to let me know, but to call me at work could get me in trouble, so instead he'll text me and I'll see it when I get a chance, and then I know I need to pick them up. We also text for other random stuff, cause we're just weird like that.
We also both text with our friends alot...we're not big phone people. We don't even have a house phone; just our cells.
But...her texting is making you uncomfortable. And that means, regardless of topic or context, the texting is inappropriate. Your husband should respect your feelings on this and tell her to stop texting him. If she won't, then he should take steps to ensure she does - block her number, change his number, report her to management for harassment, whatever it takes.
If he won't do that, then regardless of what he tells you, there is a problem there. No coworker, no friend, should ever be more important than your spouse.