What should I do if my best friend wants to commit suicde by driving his car into a river?

Trying to commit sui. Call suicidal hot line or call 911 for help if he is not willing to go to er. For evaluation and treatment.
Talking. Stay in touch, be supportive, lend a hand and try by all means to get him to call a suicide hot line or check in at a local mental health facility, er, urgen care center, pcp office or any mental healh facility. See other friends, family and support personnel to assist you.