Hello

I'm currently coaching a friend, and I'd like to know how to be more effective in coaching him.
IE as he is my friend I tend to be too gentle with him, which I guess I should change when I'm coaching him (off course staying supportive, but brutal honesty regarding mistakes seems more efficient than over-indulgence).
For sure there is loads of things to know about coaching that I haven't a clue about, but I couldn't find anything on google about coaching for poker specifically.
Could someone help me with that please ?

Thank you :).