Don't Get Roped Into Delivering Bad News on Behalf of White Cowokers or Managers
I have been placed in the uncomfortable position--more than once--of having someone White share a Black coworker's alleged performance issues with me in the hopes that I could talk some sense into the person. For instance, a White manager has asked me to speak to a Black, female coworker about her "bad attitude" because we were lunch buddies. That was totally inappropriate! You should consider any requests like that inappropriate, as well.
Relaying a message like that is not your business, should not be made your business (bad news should be confidential between coworkers or between managers and subordinates), and should come from someone in authority.
If you are ever put in the position where someone has asked you to serve as the office’s Negro anchorman (disseminating “breaking news” to other minorities), kindly tell them that, despite any existing relationship you might have with what I will call their "target," you are not comfortable being placed in the middle of a sensitive situation.
Thank the person for considering you capable of being careful and delicate in conveying sensitive information, but make it clear that you do not wish to take your relationship with your other coworker that direction. Emphasize that you are not their supervisor or manager and, therefore, it is inappropriate for you to conduct yourself as such.
You can also make it clear that you will speak to the person about the subject, but only if they choose to share the information with you first. Other than that scenario, you would rather focus on your work and personal conduct and leave any verbal warnings to the appropriate members of management.