a few choices..
the best is to disassemble the headphones and replace the 3.5mm headphone cable. you can probably even find one that is split just like the one you have now.
if the chew is below the split you can probably just put a 3.5mm end and use it as a short lead or use a 3.5mm extender cable with it but this is less than idea.
its a simple fix but it requires a few minutes of your time.