since this not a very timing critical task you can use most or any ("standard") optocoupler to do this. because they're cheap and always available where i do my shopping i swear on 4n28's.
you could even (and i tend to recommend that) build your own: connect a photo diode across your button. like that in can be "pushed" with just light, or better with a blink of an LED connected to your board.
kuk