You can ask your contact to use the unregister app here https://whispersystems.org/textsecure/unregister/ Apple offer a similar tool when leaving iOS https://selfsolve.apple.com/deregister-imessage
You can ask your contact to use the unregister app here https://whispersystems.org/textsecure/unregister/ Apple offer a similar tool when leaving iOS https://selfsolve.apple.com/deregister-imessage