He'll either settle with your insurance company, or file with his own carrier and they will subrogate (attempt to collect the money they've paid back from your carrier). Nearly everyone vastly over estimates the value of their own vehicle in the event of a total loss. Policy limits notwithstanding, no insurance company- none, not even his own is going to pay more than the actual cash value of his vehicle, which is very different to the retail price. For example, the insurance company doesn't owe blue book value or wherever he's getting his figures from. They owe what his truck has actually been selling for based on comparable vehicles sold over the past 90 days in your area.
So with that in mind, how far over the policy limits are we talking? Not that it probably matters. Unless there was an injury involved, he's very unlikely to get an attorney to touch a property damage claim with a ten foot pole. There is just no incentive for the attorney. And hiring an attorney is not beneficial to him in the event of a property damage claim without injury either. The insurance company will STILL stick with their offer, with possible room for SOME negotiation, and when it is paid out, his attorney will take a third of the settlement anyway, heh. He'll whine and wail, and eventually, he'll either settle or file with his own carrier. In the case of the latter, the subrogation process will be negotiated between the two insurance carriers internally and you won't be involved. Should the offer be over your policy limits, sure he can sue you for the remainder, but regardless he's not going to get twice the value of his truck. Ever. And he'd be an idiot to pursue that route anyway in lieu of just filing with his own carrier, who will pay his damages in full minus his deductible.
So, my advice- relax. This happens often and 9 out of 10 times the talk of lawyering up is just idle threats. Your insurance company will handle it. Source: I'm a total loss claims adjuster who deals with this every single day of my life. There's no such thing as a happy claimant. Good luck!