AFFF concentrate is mixed with water to create the "foam solution" then aireated at the nozzle to produce "finished foam". If he's using a 10% mixture ratio to create more foam, it's still 90% water. Unmetered amounts of water/foam into the intake of a diesel engine can't be good for it. When water converts to steam inside the engine it expands 1700 - 1. I'm not sure which engines can survive that compression ratio. It may not add to the combustion, but it will certainly alter the compression.
You will not get off scott free from damage in any event. The engine will most likely be hot when/if a fire starts, unless it's an electrical fire, which AFFF solution isn't going to extinguish anyway. Imagine dumping a few gallons of water on top of your hot engine. It's still better to put the fire out than it is to burn the entire motorhome, but there's gonna be some damage.