Husband didn't cheat but feels bad

I know how this is going to sound but hear me out.

My husband doesnt drink but he went out with his cousin and his cousin got speaking to a girl (who had a friend with her)

Let's call his cousin jamie.

Jamie wanted the girls number and the girl insisted she would only give hers if her friend could have my husbands.

In a moment of desperation Jamie handed this other woman my husband's number, when he had already left and was at home with me.

He got a text the next day off an unsaved number introducing herself as Katie, asking if they could go out. He immediately freaked out and showed me the message and insisted he wasn't up to no good. I was a bit dubious at first but i spoke to Jamie and he revealed he had given his number to her.

I was so angry, and upset. It's now 3 days later and my husband is so guilty, he actually cried to me thinking I'm going to leave him because he thinks I don't believe that he is innocent in all of this.

I do believe him. Of course I trust him, but Jamie has done something incredibly stupid

How do i make my husband cheer up and feel less guilty?