I don't know what to say to my friend now...?

There is a person in Tooting with some influence and he holds a grudge against me that results in him lying about me to everyone (comes down to defamation of character, but it's expensive to sue) and it actually harms me every time because it results in things not working and so on, so I told my friend in Streatham about this man with that grudge and lies and that, and asked if they could just break his nose and intimidate him, now, my mate said not quite and prefers to build a letter bomb and send his home flying. I explained that that would be serious and could result in serious injury or even death and the answer I got was "Too late, I'm already rehearsing the act" oh dear, I wish I had never told that particular friend. I could be dragged into that even though I tried to talk my friend out of taking such drastic action. What can I do to protect me if he does anything really heavy?