What do I need for facebook Audio Live streaming?
I would consider just using your phone (use Wi-Fi depending on situation) :) 

I found this great video of how to do Facebook Live Audio cast (click here).

For your situation I find it quite suitable to use your phone instead of your laptop. It will make it easier since both you and the guest are using a phone. Then it makes the whole process seamless.  

You can buy external stereo mics for phones to improve audio drastically.   

You can gradually build up a better setup over time once you get more comfortable with Facebook live. To be very good... it takes time :)

Try to have at least a 5-10mbps for a video live stream.