I'd be more concerned it would be a red flag to NOT report the amount on 1099K as gross income.  Not sure how much 1099K matching IRS is doing but why open up the client to a CP2000 notice.

I have similar situation with some of my clients that have co-ownership in horses.  The company they sell the horses through will only issue 1099s to one of the owners.  I report the gross income from the 1099 on the return with deduction on other expense line labeled  "horse sale income paid out to co-owners".