The law can vary from state to state and country to country. Ultimately, in a solo suicide, only the person who dies is liable for his/her own death. Depending on your role and what you say on the video call, you could be an accomplice. It would also depend on the aggressiveness of the police and coroner in your area. Your friend's family might also be so distraught that their response in healing is to go after you, even if you were just "there" for your friend and not encouraging him/her to die like the much-publicized case of the teen girl who was deemed responsible for encouraging her boyfriend's suicide. Add to this the potential for authorities being able to get a copy of the video call, if that's possible.
While I applaud you wanting to be there for your friend -- I think most of would like to have someone by our sides even if virtual. But I feel you're opening yourself up for some potential issues and headaches and questioning.
I'm not an attorney so I don't know the actual legal issues. This is just what my intuition tells me.