My best friend was telling me about a lump that forms somewhere on the outside of her vagina before & durning her period & it's usually sore. She's a virgin so is this something serious? It's always only one lump though

It's not related to her period. Does she shave? It sounds more like an ingrown hair, or possibly a cyst or pimple. We can't diagnose online.

She should talk to her doctor about this.