Thanisarro Bhikkhu (aka Geoffrey DeGraff) is a well known and well respected teacher in the Theravadin tradition. You are very lucky to have an opportunity to sit and learn from him.
I have read his writings, and I can tell you from my perspective, he is an arahant. You couldn't be in better hands than his.
His words and suggestions come up in my mind when I meditate, and help to understand what I am doing.
That's a very good sign. His meditation instruction is well grounded and solid advice.