I'm going to be honest with you. If your friend signs a statement and tells them it was your grow, along with his, you will get arrested to. If he signs a statement that you helped take care of them you will probably get arrested. If he tells them you had nothing to do with them you're good to go....
Sound advice.
I had 2 buddies involved with another kid in a similar situation, but it involved breaking & entering and not a grow. They questioned buddy #1, he confessed and said basically yeah me and these 2 other guys were involved (who had not been on the cops radar or even questioned before he went downtown).
I don't know the specifics of what was signed but all 3 caught charges.
If it's your friend's first run in with the law (as it was for buddy #1) it can be pretty nerve racking, and it's easy to get caught up in that "if I don't tell the truth they said they will sodomize me" mentality.