How do I stop texts from going to 2 separate iPhone numbers? (Not Group Texting)

I have a friend and her husband who both have iPhones. Whenever I text her, the text also pops up on his phone. They were sharing an Apple ID, which I suspected might be the problem, so I helped her create her own Apple ID, but it's still happening. This is not a Group MMS. It's an individual text. If I send it to her, they both get it. If I send it to him, usually only he gets it.

This has been going on for months. Both are now using the latest Apple software update, but the issue still remains.

They tried to contact Apple, but were told that without an service agreement, too bad.

Can you think of any way to fix this issue?