How hard would it be to fix the frame of a geo metro that has rusted through and broke on the drivers side. it is undriveable at this point.

Basically, you DONT fix it. It's a common problem with these cars, more so if they were used in "Rust Belt states...AKA, States that use road salt on the roads in the Winter. The cars sub frame has rusted out and is a safety hazard now, Long story short, it's a parts car now!. You often see several Geo's for sale on E-Bay with this same problem and are sold as parts cars only. Part the car out or look for a high mile (Or bad engine) 91-94 Geo Metro for sale from a Southern state or with a clean undercarriage and do a swap of parts.

it is undriveable at this point.
....Yep! Dont risk it.