Before reading the article I was sure you installed a SIP client on Raspberry Pi and wired the speaker/mic to the headset so she could actually make calls with it.
Still awesome.
There's a guy in Australia who makes little in-line adapters that translate the pulses into touch tones. It's about the size of a U.S. wall jack, line-powered, and works very well.
I've had it for something like ten years, so I'm afraid I don't have any contact information. But if he's making them, someone else might be doing the same.