You don't use an AI for tren..... AI's are for aromatising compounds such as testosterone
Your friend is not tren ready if he's asking this question, you need prolactin control
Caber or prami is what your friend needs when running a 19nor steroid such as deca/NPP/tren
I hope he is planning on running test as well as a base with the tren, and in that case aromasin will be the best bet for an AI, with arimidex coming in as a close 2nd.