The fire monitor is supposed to 'knock down' the fire. If the fire team are going to approach the aircraft to either attempt a rescue or tackle a smaller fire then the monitor is supposed to be switched off and hand held hoses used.
On our helideck we always have one member of the fire team on the monitor on the upwind side. It's checked regularly as part of our planned maintenance system and it also gets checked during the annual helideck inspection.
Having said that, I don't know if here has ever been a case in the U.K. where a helicopter has crashed and remained on the helideck, thus requiring the intervention of the fire time (not saying that you shouldn't be prepared though).