Being sued due to car accident?

[DS]

My husband was making a left hand turn in his work truck and was rear-ended by another vehicle, which was then rear ended by another vehicle. The first vehicle to hit my husband said there was smoke in the air so he suddenly couldn't see. The police sent my husband on his way right away as it was obviously not his fault. This was two years ago. Now he just received a court summons from the first vehicles court/lawyer that he is sueing my husband, my husband employer, the second cars driver and his employer. The person giving the summons said to give it to his insurance company within 20 days. But, it wasn't my husbands fault and it wasn't my husbands vehicle, it was his work vehicle. Not sure what to do. If he doesn't respond he is considered guilty and will be charged to pay the $500,000 they want.
My husband was not given a ticket and his work truck was brand new and insured.