If someone says "be right back," it's a common way of indicating that they will temporarily leave the conversation or activity they are engaged in, but they intend to return shortly. In response, you can acknowledge their statement and indicate that you will be waiting for their return. Here are some appropriate responses:
"Sure, take your time. I'll be here when you get back."
"No problem, I'll wait for you."
"Okay, see you when you're back."
"Alright, I'll catch up with you later."
"Take your time, I'll be around."
Remember to be understanding and patient, as the person might need a brief break or have something important to attend to before coming back to continue the conversation or activity.