There was some evidence in the past that relaxation techniques such as autogenic training can help people with asthma. However recent study reviews confronted that. This does not mean that autogenic training is of no use. Nevertheless, if relaxation did not help with asthma, scientists would not research that possibility in the first place.
Stress itself is not good for asthma and practicing relaxation showed some improvement in that direction. Since practicing relaxation has many positive effects on wellbeing it is wise to include autogenic training or meditation in a daily healthy lifestyle of a person with asthma.