Can you ask what size the mattress is? I think it may vary from cab to cab.
When DH had the truck, it had an x-long twin mattress.
Is your BFF thin or round? If thin, maybe only 54 inches wide by about 90 inches long. The extra length to tuck in or to pull over his head if it's really cold.
Don't make it any wider than necessary, because those bunks are really hard to make up nicely.