If someone says "be right back," it generally means they need to step away temporarily and will return shortly. You can acknowledge their message and let them know you'll be here when they come back. Here are some appropriate responses:
"Sure, take your time! I'll be here when you get back."
"No problem, I'll wait for you. Just let me know when you're back."
"Take all the time you need. I'll hang around until you return."
"Alright, I'll be patient. See you when you're back!"
"Take your break, I'll still be here when you return."
These responses show understanding and patience while respecting the other person's need to step away temporarily. It's a polite way of acknowledging their message and letting them know that you'll be available for continued conversation or interaction upon their return.