Tell them to pound sand. It will never be just "his problem". Does he even know how to do that? Sounds like no.
But you're off on the wrong foot with a digipot anyway. A DAC is the way to go, you'll need a post amplifier to raise the 0-5V to 0-12V. If you can tolerate very slow action, you can low pass filter an Arduino PWM output to get a pseudo analog signal that could drive an amplifier.
Actually, in that case, you could combine an amplifier (e.g. MOSFET) with a low pass filter in order to do that.