How to isolate heat from oven towards refrigerator?
A metal sheet, suspended between the surface of the stove and the fridge, would be safest, simplest and cheapest. Consider, though, how long the payback would be to the investment ROI): how often does the oven run, how much does it raise the temperature of the side of the 'fridge, and how much does electricity cost?

From the environmental standpoint, consider the energy used in fabricating a sheet of aluminum, and how much energy is saved.

For safety, do not apply insulation in a manner that would cause more heat to accumulate on a wall behind or next to the oven.