Apologies, yes can be treated, but once damage occurs the damage can't be undone eg once blind, always blind, far as I understand it.
Anyone that has unprotected sex with someone that has multiple partners should read up on syphilis. Might think differently after.
https://my.clevelandclinic.org/health/diseases/4622-syphilis
Syphilis can progress through four different stages. The infection causes different symptoms in each stage. People are very contagious in the first and second stages and can easily pass the infection to their sex partners. The stages of syphilis are: primary, secondary, latent and late (tertiary) syphilis.