Husband gets very inappropriate texts from female coworker- daughter sees it first- tells her "who knows what I do when I am out of town"- deletes messages, but gets on speaker phone with daughter in room and tells her never to text him again. Daughter upset-thinking I might be cheating on her dad- at least that was what he alluded to so attention diverted from him. I think this person has crossed the line and a door was opened to allow this kind of comment to be made. He admits she has a crush on him- what to think or to do.

You need to be more clear. I read this a number of times and I'm not completely sure what I'm reading. How did it come up that you may be cheating?

Yes, is someone cheating,

What does daughter have to do with it,

I am confused.

But a person can not stop another from texting them, if they get a bad text but tell the person to stop, please slow down and give the story as if we have no idea ( we don't)

Agreed, your story is foggy as written.
Is this it?
Your husband goes out of town on work, but when home, your daughter sees a text on his phone and confronts him. He says to her 'who knows what your mother does when I'm out of town?' He also calls the co-worker on speaker phone in presence of daughter, and tells co-worker to never text him again.

It was immature and selfish of your husband to put your daughter in this position (how old is she) as well as accusing you to cover himself. But you are critical of the 'other woman' and should be dealing only with your husband. This is between you two! Communicate. Try to find a time when you can talk with him as calmly as possible. Be prepared for having to tell him that if you ever have proof of adultery, you are one step away from the divorce lawyer's door.
He may be engaging in 'just talk' even if inappropriate, or they may be having an affair, you don't know. You just have to keep your eyes and ears open. Or even hire a PI.