Fine!

I outlined the cheapest mitigation elsewhere in this thread - a propellant depot or other human activity at L1.

The Martian atmosphere that our friend doesn't believe in takes care of most radiation already. A meter of regolith over habitations does the rest, for humans.

The radiation levels are already tolerable for crop plants, and we can grow them in 0.1 Bar and millibars of O2.

Now the absence of a global magnetic field's effect on human health is quite unknown. It may be a nothingburger, or we may need to set up magnets in habitats, but we don't have the required data.