Taking your question at face value, I notice you actually don't mention recording as a requirement. This may have been a simple oversight, but on the other hand it could be because your friend does not value the forensic value of the camera much, and he might consider any scenario in which the tools are actually stolen to be a complete failure. This is understandable since, even with the best footage possible recorded, he might not know much more than the precise time and the general make/ build car/perp, and this only hours after the fact, leading to a low percentage of recovery.
Nor do you mention wanting the camera for the mere deterrence value of its presence explicitly, although one would imagine that any such deterrence would be welcome since it might prevent the theft in the first place, as opposed to after the fact forensics. But, primarily, I think you do actually mean live monitoring when as you say:
Is there any kind of camera he could use at the back of his car to monitor the trailer throughout the night?
Even though one might be tempted to say that your friend, 'doesn't really want that...', since it hardly seems likely that your friend will really 'monitor the trailer throughout the night', my guess would be that you mean POMM, Peace of Mind Monitoring. POMM is rarely seen in commercial settings, but is often seen in residential settings, and most commonly found in the humblest of all surveillence systems, the baby monitor.
So, I believe your friend's intended use goes something like this:
Hear strange noise, check tool cam (without getting out of bed)
Hear alarm go off, check tool cam
Hear talking, check tool cam
Feel uneasy, check tool cam
This state of mind is due to the previous theft of 'locked tools' and is understandable. For the record though, I am not recommending a baby monitor necessarily, just saying the mode is the same. Also, although, not foremost in your friends mind, I think he will eventually want recorded video, although more to review suspicious incidents that he missed live, then for prosecution.
Luke, indicate if you agree with my assesment or if I'm clueless. I'm looking at some actual options now, but would like to know if I'm on the right path before making more of a fool out of myself than I already have....Also, what's the rough budget?