Is it permissible to hand over the death certificate of the first wife to her husband so that he can claim a pension for her, even though she was not insured by the employer?

If the wife has indeed passed away, there is no objection to giving the husband the death certificate for him to obtain a legitimate right. However, if he will use the certificate fraudulently to obtain an illegitimate right, and you are aware of that, then do not assist him in sin, in accordance with the verse: "And do not cooperate in sin and aggression" (Al-Ma'idah: 2).