Section 8 of the Matrimonial Causes Act:
"8.(1) A petition for divorce may be presented to the court Grounds of either by the husband or the wife on the ground that the respondent -
(a) has since the celebration of the marriage committed adultery; or
(b) has deserted the petitioner without cause for a period of at least three years immediately preceding the presentation of the petition; or
(c) has since the celebration of the marriage treated the petitioner with cruelty; or
(d) is incurably of unsound mind and has been continuously under care and treatment for a period of at least five years immediately preceding the presentation of the petition,
and by the wife on the ground that her husband has, since the celebration of the marriage, been guilty of rape, sodomy or bestiality.
(2) For the purposes of subsection (1) of this section, a person of unsound mind shall be deemed to be under care and treatment while he is detained, whether in Kenya or else- where, in an institution duly recognized by the Government as an institution for the care and treatment of insane persons lunatics or mental defectives, or is detained as a criminal lunatic under any law for the time being in force in Kenya; and a certificate under the hand of the Minister that any place is a duly recognized institution for the purpose of this section shall be receivable in all courts as conclusive evidence of that fact."
Clarity....aaaaahhhh!!! Just petition for divorce under one of the grounds above, presumably (b) and perhaps (c) too.
The absence of the marriage cert is something that can be explained through a statutory declaration or oath or other documents to be filed when petitioning for divorce.
Oh, and hopefully its not that the guy is insane but not strictly speaking not in a mental institution coz he must be in such an institution for at least 5 yrs for this to be a ground for dvorce.(check subsec 2)