How do I proceed if a friend of mine suddenly have problem breathing?

Call 911 ambulance. Anytime one finds a person who cannot breathe properly, and cannot tell why, one should right away call 911 for an ambulance. If the victim is choking and cannot speak, the heimlich maneuver can help. If the victim is having an asthma attack, there may be an inhaler in the pockets or purse. If the person is having an allergic reaction, there may be an epi-pen there, etc...