Why would putting charcoal in her vagina (which seems to be a kind of extreme thing to do all by itself) make her pee black? People don't pee from their vagina. I don't generally think douche mixes are safe, it is better to let the body clean itself out. If she does douche, things could get even more messy in the vagina than they are now, but possibly sterile water (boiled and cooled, and if she has no access to a perfectly sterile applicator, she should see if she can get a perfectly sterile syringe). The reason to be so careful that everything is sterile is that she would be needing to avoid adding more bacteria or wrong substances into an area that already has a lot going wrong. She should also bathe and be sure the outer area is perfectly clean if she does try syringing water in to rinse the charcoal out. Frankly, I think she should see the doctor about this and not mess with any more home tries.
Please advise her not to douche. Given the fact that she has a bacterial infection, she can push the bacteria further into her reproductive area causing pelvic inflammatory disease, which can be quite serious. The charcoal is very messy and is coming out of her vagina when she urinates. Urine comes from the urethra, so no charcoal is coming from the urine itself. She needs to seek treatment from a healthcare provider. A simple wet vaginal wet mount will give the provider all the information they need to appropriately treat the problem/infection she has. I would have her call her medical provider on Monday. I hope she feels better soon.
It is accurate that urine does NOT come out of the vagina but seeing as there was a severe infection to begin with, there may be a possibility of a rupture somewhere that is allowing urine to leak through. This is only if, and truly if the urine is coming out of the vaginal opening which is unlikely. Either way, a DOCTOR (preferably a gynecologist or urologist) is essential here. Please let us know the outcome so we can learn.