Could be an STD but more likely to be bacterial vaginosis. It’s a very common infection that can happen when a woman’s PH is thrown off. PH can be thrown off for a host of reasons such such as sex/semen, menstrual cycle, diet, medication, etc. It can actually be thrown off for using the wrong type of cleanser down there. It’s symptomless in many women and the body can clear it on its own, but sometimes medication is required.
Most of the time it happens for reasons completely unrelated to being “dirty”, but in this case she should have refrained from sex until she got that cleared up.