you can even through your own insurance!

i would be on the phone to his insurance and tell them you wish to make a claim agains their insured, they will then send you a claim form or do it over the phone and tel lyou to get a quote for a NEW mirror and supply them with the quote. Dont be pushed nto taking it to their approved garage either tell them that it is going to be done by BMW (do you have all your work done at BMW or does it have any BMW history with it? as that will provide more leverage against them if they get funny)

to be honest if i knocked a mirror off i would prob cough up the cash and save going through insurance as its cheaper than the excess prob