Setting a Pin to HIGH to simulate a button with a "pull up" resistor
First of all, I don't think you will need full simulation of a button. There are a couple of things this approach won't have:


Bouncing: it'll be (kinda, minus interference) a square wave. With a real button, the metal contacts "bounce," thus creating a weird waveform. Depending on interference, you'll still need debouncing code, although I would be surprised if it was necessary. I'd inlcude it in case I modified it to add a real button and/or there's some weird interference.
The time it's "pushed" may be very short, depending on your code. Keep this in mind when designing this.


Basically, you're hooking up the Arduino and the RPi together, with a voltage divider or a level shifter between the two.

Voltage divider:


  


Sparkfun

That is a voltage regulator (drawn four different ways; it's the same circuit). I calculated the values you will need: 1700 ohm for R1, 3300 ohm for R2.

Of course, serial (with USB of the Arduino) communication would be a lot easier...