I suffer from very mild asthma so fortunately it's well under control.
Firstly, don't panic. Yes, unbelievably hard to do when someone can't breathe. Reassure them that medical help is on the way. Call for an ambulance ASAP. Do not delay this action.
However, it seems to be a natural thing for people to want to pat people on the back if they can't breathe. Believe me, this is one of the worst things you can do. Their lungs are already in spasm and to irritate them will just make it worse.
Bluey,
Since you're often in a stressful environment, that can trigger first time attacks. Attacks will look like wheezing or coughing. The cough won't let up and it's a real hassle trying to breathe. Again, if it's the first time, the victim will be very stressed and panicking about not being able to breathe.
Ask if any one has some ventolin and see if that works.. if it's apparent it's not helping, call an ambulance.