@Nori
Deleted contacts and threads multiple times. Apparently our other friend’s single thread works just fine. They can text both numbers on the same thread. But they have an iPhone 16, and I have a 15 Pro Max, for what it’s worth.
Tell them to de-register their number from iCloud. Should work. And if not, try turning your MMS on and off. If yours is SMS, try switching to MMS.
Make a Reddit post, indeed you did.
Completely remove the Android number from your phone, including in the iPhone contact info. Then add a new contact with a different name, and add the Android number to that.
Haven’t heard of anyone using text messages on Android.