Sorry to hear of the accident, glad your dad wasn't seriously injured and I hope so for the moron that turned in front of him as well.
Depending on how much he turned prior to the impact, that may have precluded deployment of the airbag. There's a reference in one of these posts to crush sensors, and that's exactly what's up there, so if you swerve and only damage one corner or another, it's not a "straight-on" impact, so less likely to affect the sensor and deploy the airbag. (The more the front end deforms, the more likely the sensor will deploy the airbag.)
Would he have been better off not to steer and hit solidly? If the goal is to deploy the airbag, certainly. However, my rule of thumb is a glancing blow is almost always better than a solid, straight-on hit. Of course that's only for motor vehicle accidents: there is good evidence that had the Titanic
hit the iceberg dead-on, it would have been seriously damaged, but stayed afloat. The glancing blow allowed the 'berg to tear into several different watertight compartments, so there a glancing blow isn't better. But if you're dodging icebergs in an ML, you've already got much greater problems.
Take care and enjoy the ride,