I can only recommend your friend see a good general doctor and possibly a specialist. My mum has/had chronic bronchitis after life-threatening pneumonia - and she finally changed doctors under pressure from her two daughters

and he put her on treatment plan which involved steriods for about three weeks from quite a high dose, but weaned off quickly, to reduce inflammation, then once they could actually see what was going on properly, he could work out whether there was an underlying infection, asthma or another problem.
End point is that she is much much better now, although we can always tell if she's coming down with a cold because the first thing she does is cough and cough (seriously, I worried she had lung cancer or something)...