Lost my 18 year old son 3 years ago in an accident. As others have stated, there’s nothing you can say or do to make the pain go away. But just being there, letting them know you care and walking through it with them is important.
Also, it’s important to be there not just in the hours and days following the accident, but in the weeks and months ahead. In most cases, the lowest point for them has yet to occur. When the quiet settles in and everyone goes back to their life when yours has been forever altered, it can get to be very lonely. Being there to let them know they’re not forgotten goes a long way.