I find it strange how your boss has confided in you about a possible high profile termination, and informed you that you will take over?!
Whatever happened to having a proper investigation, mitigating circumstances and then depending on evidence you have to support to terminate?
It was irresponsible of him to tell you in the first place, and it certainly is not your position to “warn” your friend.
If i were in your situation ( i would be looking for another job) play dumb, like you never had an inkling, and then check out the classifieds.