If they still see work together, I suggest you put a voice-activated recorder in his car for a week or two to see if he is talking with her. There is no way you could monitor just by looking at his phone. There are hundreds of apps that would never show logs. Apps can be deleted and reinstalled and deleted again.
Did you have the truth from him? Probably not, he would be the first cheater I know of who came clean 100%. The fact that other woman told you she initiated likewise is not reliable. That is the problem with cheaters and liars, you can't tell if they are lying or being truthful.
Add on that he deleted all evidence. He deleted all texts, all messages. Do honest people usually destroy evidence? Maybe he deleted because he was afraid, but destroying evidence is the hallmark of a liar who is trying not to get found out the truth.
The important thing, though, is to regain trust. The voice-activated recorder can do that. If you check on that for a week or two, and no conversations or sitting in cars with her and him, your trust increases. If you find out they are talking or together, then at least you don't waste any more time.
The heart of cheating is breaking a promise. Why did he break his promise? What did he say to himself that he would say, "It's ok for me to cheat on my wife." He did it once, then over and over again. If the store was opened on Christmas Eve, when would that affair have ended?
Do you think other employees would suspect he was having an affair? Now supposedly he doesn't talk with her and avoids her, so do you think the other employees gossip and wonder why? Do you think at least one of the other employees ever ask your husband that, what went on between him and his bestie? If there was gossip or questions like that, would your husband tell you about it? In the past month, let's say a couple of weeks at least given the holidays and vacation, there likely would be some type of incident at work related to her and him, whether good or bad. Maybe she approached him to say sorry. Maybe she say she loved him. Maybe an employee approached husband to ask what's up. If any of that happened, would you want to know?
I think the voice-activated recorder can help you catch him being honest, or catch him being dishonest.