Don't wait until another two cats meet the same fate, call the RSPCA and have a chat with them. Stay anonymous if you want. They will just go and chat with her and maybe their visit will scare her into thinking twice about "getting fed up" with another two cats.
I've been visited by the RSPCA. Someone called them after I'd rescued a colony of eight feral cats who needed help. Apparently, the caller had assumed there was no one in the house caring for them. I had ten cats in total, counting my two domestics. The RSPCA woman had a friendly chat, saw that the cats were well cared for, gave me a cat tree, supplies of food and neutering vouchers for the ferals, which I was grateful for.
They weren't ogres, just one soft-spoken woman who assessed the situation. (Six of the eight found good homes and I kept two, one of whom is sitting on my lap as I type this: Fred!)
So just get on the blower. It won't hurt.