As she concedes, the marital disqualification rule “is one of disqualification and not of privilege,” and therefore concerns a party's competence to testify. See Com. v. Azar, 32 Mass. App. Ct. 290, 304 (1992). It is settled that "[t]he mere fact that a party may not be mentally competent to testify is not a sufficient reason to prohibit the other party from taking his deposition." Dang ex rel. Dang v. Eslinger, No. 14 Civ. 37, 2014 WL 3611324, at *2 (M.D. Fla. July 22, 2014) (citing cases). This is due, at least in part, to the fact that even though an incompetent witness’s testimony “might not be admissible at trial, his deposition could lead to admissible evidence.” See Fjellman v. Forest Hill Co-op., No. 06 Civ. 14470, 2007 WL 1806173, at *3 (E.D. Mich. June 21, 2007); see also Bucher v. Richardson Hosp. Auth., 160 F.R.D. 88, 93 (N.D. Tex. 1994) (“[P]laintiff is not entitled to quash the deposition merely because J.B. may be incompetent to testify at trial. The right to depose a witness and the right to use that testimony in court are separate and distinct.”) (citing United States v. International Business Machines Corp., 90 F.R.D. 377, 381 n.7 (S.D.N.Y. 1981)).6
The judge also argues that the undue burden argument fails because Cosby fails to show any undue burden.
I also found this 1990 Massachusetts case which covers the same issue, comes to the same conclusion, but with different reasoning. This decision hinged on the business relationship between the spouses saying:
In the present case, the court concludes that business communications between spouses acting solely in the capacity of employer and employee are not protected conversation under G.L. c. 233, § 20. The court relies on the fact that when a conversation occurs in such circumstances, the marital relationship is only incidental to the employer-employee relationship. The two speakers, acting in their professional capacities, do not fit the terms “husband and wife” in an important sense. Moreover, excluding conversations made in an employer-employee relationship would not further most of the policy reasons behind the statute. Therefore, G.L. c. 233, § 20 does not disqualify purely business conversations made by husband and wife when they are acting as employer and employee and where their marital relationship is incidental to the conversation. http://masslawyersweekly.com/fulltext-opinions/1990/01/01/anderson-v-barrera-et-al/
And I'm surprised Hennessey didn't cite this case as well, since Cosby's wife served as his business manager as well.