her messages and mine are connected how do we fix

Why did you mark it as solved if you thought it was nonsense? Are you sharing an Appel ID?

And in clearing the old phone did you do everything listed here: What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

I'm not sure what deactivating from AT&T and all the rest in your answer is about. Apple warns it is a bad idea to share Apple IDs and not just for messaging reasons, there are other problems you will run into if you are doing that. If your friend is a good Apple employee they will also tell you that.

You need to quit sharing an Apple ID. If you want to share purchases, set up Family Sharing.

Family Sharing - Apple

You're clearly confused. The fact you own the phone has NOTHING to do with the Apple ID used on the phone. The reason you are receiving messages on both phones is you are using the same Apple ID on both phones. Demo gave you the correct answer. If you choose to not believe him that's on you.

Wow, what does the fact that you own the phone have to do with anything? You can do whatever makes you happy. I don’t care.

I know some one who works for apple I’ll just ask them and delete this I’m done don’t have time for nonsense.

What makes you think we do?