There are a few solutions:

For one, an XOR gate with an inverted "B" input, the B input being the microcontroller;

What an XOR does is it turns on when inputs are inequal.

Here's the truth table below:

A0 B0 = OUT 0

A0 B1 = OUT 1

A1 B0 = OUT 1

A1 B1 = OUT 0

However, you might say, that would mean that the input from the board would always have to be on. You would be right; Hence why you'd need an inverter on B.

So then the logic would be something like:

A1 B1 = 0 (Input B is off but inverted, and A is still a working power source.)

A1 B0 = 1 (Input B is now on but inverted, meaning A and B are unequal.)

And any "A0" would just mean the batteries were dead and not be a problem.

I don't know if this solution would work as far as voltage/amperage on the XOR gate.

SOLUTION 2: Use the microcontroller to handle the current itself.

Either use the microcontroller as the way of powering the laser along with a logic level converter (You could even PWM control it!)

tl;dr

use an xor gate or a logic level converter and your arduino