I can only attest to what I would do in this situation - my girlfriend and I live in a flat that I bought and own while we both save for a house we plan to buy together in the near future. Anything property related (windows, carpets, doors etc..) I purchase myself as it's my property.
However, If we co-owned, co-rented (as you do) or something I would pay for half as we have equal stake in the property, I'd see it the same as if she accidentally broke the fridge or an appliance or something.
So personally, I'd help pay the bill, but I do understand why you feel you shouldn't have to, if it really was just dumb luck and old windows, it could have just as easily been you that broke that window and you'd appreciate the help I'm sure - NAH