A lot of the answer depends on where the hospital is, and how hard it would be for them to attract new doctors. But on the whole, sadly, your friend probably doesn't have much leverage and would find himself out of a job if he told them no. EM is becoming quite saturated (as has been thoroughly discussed on this forum), and the current state of affairs has accelerated the problem/ made it worse. Most places right now are overstaffed and would have no problem getting rid of a few people. Will volumes eventually return to normal? Maybe. Or maybe a certain percentage of the population now realizes they don't need the ED for everything and will never return. In any case, a hospital can easily afford to lose a couple of ED docs right now and when/if volumes return it won't be hard to cover their shifts with a bit of OT for a while (which the remaining docs will be desperate to pick up), while easily hiring from the large EM applicant pool in the meantime.
I keep a couple of locums contacts, and right now the "best" offers they're making that I'm seeing are for jobs paying between $150-$180 an hour for horrible places with terrible support and mostly nights. One of my former residents who lost their locums gig (hospital didn't need them any more) just took a desperation job at a sleepy rural place for $90/hr because that's all she could find to offer FT hours. It's really hard out there, and this isn't the time to put your job at risk. I'd like to give you a different answer, but unless EM docs come together as a whole (which let's be real, isn't happening), then this is not the time to do anything but bend over and take it.
Full disclosure, my hospital has now taken about $50,000 away from me for this year and I've bent over and taken it. So my money (or lack thereof) is where my mouth is.