Masturbating and rubbing are not ways to transmit STDs, as many of the parasites that cause them are not developed to live outside the body and they do not survive for so long. Is true that if you touch lesions caused by herpes and syphilis (blister and chancre) there can be transmission, but that does not mean that the rash she/he had on her/his legs were due tu any of those infections, as a rash on the skin can be caused by so many different agents.
Regarding the presence of sexual fluids, it is normal to have traces of them all over the body when holding a mutual masturbation but as I said before, that does not really change the situation, which was a non-risky behavior.
Rubbing genitals can spread some ecto-parasites that live on our skin and body air. But they are not consider as STDs, and can be spread through so many other ways.
I hope that helps.
All the best.
Rubbing your genitals together can transmit herpes, syphilis and HPV, depending on how much friction was involved. These STDs are spread by skin-to-skin contact (specifically, genital to genital skin) and don't require penetration to spread.
If there was some grinding, or a lot of friction with your penises, it's possible that one of these could transmit. Without any sores, it's not possible to spread syphilis, so if she had no sores, you don't have to worry about syphilis (and her rash doesn't sound like syphilis).
Without symptoms, herpes and HPV can still be transmitted. It's not as likely for herpes, but still possible, and there are strains of HPV that never show symptoms.
There is no test for men for HPV, so if you got that and don't get symptoms, you'll never know but chances are you'll get rid of it on your own, and 90% of sexually active people will have it at least once in their lifetimes.
I wouldn't overly worry about herpes. If you want, you can test for it now, and again at 4 months, with a type specific IgG blood test, but without symptoms by a month or so, I'd let it go.