Never had to submit a single proof of purchase other than a picture. They know if the warranty is valid by the serial # (always wise to check with MF before purchasing) and if they think something is amiss, they will request that they box be sent in for examination (very, very rare..usually only with shitty pics). If you want to double-check, put in a ticket on their website and ask for a serial number check...within 48 hours you will know if the warranty is still valid. Only way it wouldn't be is if your friend picked it up from an unauthorized seller or the MFLB has already been warrantied. Good Luck!