Help... any way to save someone once they've already had sex with someone with herpes

Anon - There's no judgment here, our purpose is to provide health information. You will find extensive information about genital herpes on this page: https://www.empowher.com/condition/genital-herpes/definition

Yes, it is possible that you have given herpes to your partner, or that you could do so in the future. The herpes virus is transmitted from skin-to-skin contact, especially in places that are warm and moist. The virus enters your body through a cut or opening in the skin or through mucous membranes—the moist inner lining of the urinary tract (in the vaginal area) or the digestive system that includes the mouth, esophagus, stomach, intestines, and anus. Then the virus stays in the nerve cells of your body. The virus is still there, even if you don’t have any symptoms or signs of genital herpes. Genital herpes is a chronic, life-long infection with symptoms that will come and go (be active and inactive) throughout your life.

The following link takes you to resources that can help you in dealing with your situation.
https://www.empowher.com/media/reference/resource-guide-genital-herpes

Good luck to you, let us know if this helps!
Pat