The Delaware Court of Chancery on Friday ruled that a top lawyer for William I. Koch will be allowed to testify on behalf of the billionaire in a bitter fight over investor rights that could determine the future of Koch's Oxbow Carbon firm.

In a 15-page memorandum opinion, Vice Chancellor J. Travis Laster ruled that the testimony of R. Robert Popeo, a Boston-based senior partner with Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, would not violate the witness-as-advocate rule, an ethical principle that generally bars an attorney from testifying for or against his or her client.

Investors from Crestview Partners had pushed Laster to preclude Popeo's testimony after a five-day trial in mid-July, arguing, among other things, that Popeo's longtime status as the head litigator on the case would taint his statements as a rebuttal witness.