Can he work as a typist in a law office that defends criminals?

Working in
the field of law may be permissible or it may be haraam, depending on the
situation. If the lawyers support those who have been wronged and strive to
have their rights restored, then it is permissible, and even mustahabb. But
if it is to support falsehood and help in wrongdoing, and is based on
telling lies and giving false witness, then it is haraam. See also question
no. 9496. 

The ruling
on working in law offices, whether typing or anything else, is to be based
on the principle stated above, because typing is something the lawyers need.
If the lawyer’s work is permissible, then the means that help with it are
also permissible, and vice versa. Allaah says (interpretation of the
meaning): 

“Help you
one another in Al‑Birr and At‑Taqwa (virtue, righteousness and piety); but
do not help one another in sin and transgression. And fear Allaah. Verily,
Allaah is Severe in punishment”

[al-Maa’idah 5:2]

If you know
that the lawyers are defending criminals, and you are asked to type up their
reports, it is not permissible for you to do that, because that is helping
in sin and approving of evil. The Prophet (peace and blessings of
Allaah be upon him) said: “Whoever among you sees an evil action, let him
change it with his hand (by taking action); if he cannot, then with his
tongue (by speaking out); and if he cannot, then with his heart (by hating
it and feeling it is wrong), and that is the weakest of faith.” Narrated by
Muslim (49). 

Defending
criminals, covering up for them and supporting them is a transgression
against the rights of the ummah, because the spread of crime and the lack of
punishment have bad consequences as is obvious, and it is a crime against
the victim in cases that have to do with specific individuals. 

Any kind of
help to do wrong or commit crime is haraam, whether it is by preparing
materials, typing them up, defending them, etc. 

What you
have to do is advise these people, and refrain from typing up anything that
you know is defending falsehood.  If they respond, all well and good,
otherwise look for another job which is free of sin and through which you
may earn halaal provision. 

We ask
Allaah to guide us and you. 

And Allaah
knows best.