Sms not working

when I send an sms to A... it now merged to person B contact ? Not blue iMessages ... green SMS. They do not share the same Apple ID. But person A phone USED TO be person B phone so that APPLE ID was used on the device previously

so now I can not sent person A a message... it will just go to person B

can anyone assist as to why? And how can I stop it

person A has been using this phone for nearly a year.. and only has started happening this week