Isn't noise just displaced air? If by "air displacement" you mean the Archimedean principle that the air in the space now occupied by the deployed airbags has to go somewhere, then I suppose that it might result in an increase in air pressure within the cabin. But would this be enough to cause injury? The cabin is not a sealed unit and the air would be forced out through the gaps. Surely any increase in pressure due to displacement would be insignificant compared with that caused by the noise, similar in volume to a firearm. Would opening the windows have made any significant difference?

I hope your friend recovers. As a hearing aid wearer myself (age-related but no doubt exacerbated by 40 years of playing with PA) I sympathise. Modern hearing aids usually have a "music" setting, but this is intended for ordinary listening and may not meet the specialist needs of a musician. Your friend may need to find an audiologist who specialises in working with musicians and understands how to tune them to meet the wearer's precise needs.