Mich. Comp. Laws § 600.1438: Oath by mental incompetent.

Sec. 1438.

Whenever any pleading is required to be verified by the party, or accompanied by the affidavit of the party, or whenever any other oath is required in order that the party may sue or be sued, and the party is, or is alleged to be, mentally incompetent, such incompetency shall not bar the administration of the oath or affirmation for the purpose of allowing the incompetent party to sue or be sued.

Instead of, or in addition to, the verification or affidavit by the party, the guardian, guardian ad litem, or next friend may make the verification or affidavit, and may do so on information and belief.

This section does not affect the competency of the witness to testify, nor does it affect the civil or criminal liability of the party for his statements under oath.