How do I get in touch with and warn a specific person?

Throwaway account. I'm not a UK citizen.

I need to get in contact with a woman who I fear is in danger from her relative. They have shared some alarming and vile sexual fantasies. I have an image, age and general understanding of their family buildup. She's not the only unknowing victim in this family. Had to play along to get this info, how do I let her know without sharing her photo?