Foot cut through to bone, and bone broken. How serious can this be?

Extremely. This is an open fracture and can lead to severe infection of skin and bone. It needs to be treated immediately.
Infection Precaution. This type of injury requires tetanus immunization. May require debridement/cleaning out of wound and close follow-up care for posssible antibiotic regiment to help prevent bone infection, osteomyelitis.