"Pretty sure" is good enough for your client to fork over $20,000 or so to IRS?  On the other hand, if he's a retired federal employee looking to collect enough quarters of coverage, he might be upset if you told him not to pay it.  Unfortunately, this is a "facts and circumstances" question.  Your hair might turn gray from worrying about such gray areas.  There's an interesting discussion here:

https://lawprofessors.typepad.com/agriculturallaw/2017/05/self-employment-tax-on-farming-activity-of...

I wonder what the 1065 instructions say about this.  My guess is that they are wishy-washy, because that's usually the case with SE tax issues.