Basis in a partnership starts with cost. Then you add the income, subtract the deductions, etc. to arrive at basis.

The 1099B likely shows original cost.

If these are Publicly Traded Partnerships, the partnership provides info on how to calculate the basis and how to report the ordinary income and capital gain portions of the sale.

And you have to report all the stuff on the K-1.

And tell your client not to buy and sell these. Your fee for dealing with them is more than he is likely to make off them.