Spouse has strep with a rash, scarlet fever. How serious/contagious is that?

Very. Strep is quite contagious; but after a few days of an effective antibiotic therapy, the contagion risk drops dramatically.
Contagious. Prompt treatment of strep throat with antibiotics prevent the .Adverse consequences that can occur (abscess behind the tonsils, rheumatic fever, valvular heart disease). Generally, kids can go back to school 24h after starting antibiotics (and no fever). Prudent to avoid intimate contact (don't swap spit, eat from each others plate, etc) for duration of antibiotic rx.