Group Texting Problem

All of our users are able to participate in group messages with both RC and non-RC numbers except for 1. She has trouble receiving messages and is unable to send messages in a group. When she tries to sent a text in the group, the text is immediately highlighted in red and an error message pops up saying "Message is not sent. Recipient extension is not found." We recently moved to RC from another phone company mostly for its ability to host group texts. It is crucial to our communication with all of our clients and partners that this issue is solved quickly.