Forcing Contacts to use someone’s mobile # in existing group chat threads

I am in a group of friends who have an existing group text chat thread (not all of which are iPhones). One of our friends, called Dave (who does use an iPhone)has both a mobile and a home phone (the latter of which doesn’t accept text messages). At some point (not sure when or how), Dave’s home number had been set by at least one of us in their Contacts without defining it specifically as a mobile #. Hence, when it was used within the group text, Dave didn’t receive it. As such, now whenever subsequent group texts are sent, they are being sent to his home phone. Even though we all have removed his home number from our individual contacts for him, the group text seems to “remember” (and therefore uses) his home number.

How can we get the group text to start using his mobile number only?