I'm not a lawyer, I just play one on TV, but you have a case because of a term called merchantability:
The following information is for entertainment purposes ONLY.
Merchantable adj. a product of a high enough quality to make it fit for sale/rental. To be merchantable an article for sale/rent must be usable for the purpose it is made. It must be of average worth (not necessarily special) in the marketplace and must not be broken, unworkable, damaged, contaminated or flawed.
The rental product was FLAWED for the intended use. Being flawed can be determined by looking at all the service logs and rental contracts for that piece of equipment.
An item is deemed merchantable if it is reasonably fit for the ordinary purposes for which such products are manufactured and sold. For example, soap is merchantable if it cleans. In general, a seller or manufacturer is required by law to make products of merchantable quality. In the event that the items do not meet with the proper standards, a suit can be brought against the seller or manufacturer by anyone who is injured as a result.
Ask the rental agency if they can supply your lawyer with all the logs and service contracts along with all the customers that rented that equipment so you can contact them all...since day one. Chances are the product outlived it's merchantability years ago.
OH and ask him when the last time he has all the welds x-ray'd and the tests for the metallurgy reports.
Do you think he will want to do that?