The airbag unit is attached to the steering wheel with two screw (at the back), then you just disconnect the small thin white (or read) lead. nothing complex as far as I know. But sounds like they just haven't disconnected the battery 1st for 5 or 10 minutes before unplugging it, which then registers it with a fault.
Not sure how it's reset, but If the garage are unable to sort it, an airbag or auto electrical place should do it for not a lot, if you tell em whats happened.