My client and her client are sent to different rooms

I have a client who uses the basic version of Zoom. Sometimes when she schedules a meeting with a client, she and the client will wind up in different Zoom rooms and cannot connect with one another. She sends a link to the client either through email or Facebook Messenger and then, when the time for the meeting arrives, she clicks the Start Meeting button and goes into a Zoom Room. Her client clicks on the link that she was sent in email/FB Messenger and goes into a different room.

Any idea what causes this or how to fix it?