Suggest Treatment For Mitral Regurgitation And Fainting Spells

A friend of mine has mitral regurgitation. The other day he had a fainting spell. When he came to he insisted we cancel the ambulance. I want to know if there is anything I should or should not be doing if this happens again. How long can he be unconscious before it is absolutely necessary to call 911. Can I administer CPR if his heart stops, or he stops breathing. Or would this put stress on his heart?