You don't say where you're posting from.
I'm not sure where you got the term "fiduciary custodian" (never saw those words combined).
Do you have a power of attorney from the person in question? A power of attorney wouldn't normally enable you to appear or file suit on behalf of another individual, as that would involve the practice of law (though a POA could enable you to engage an attorney for the person).
Presumably an adult, one would seek-have *conservatorship* over his-her person and all affairs (though they may use the term "guardian" where you are, normally that would be with regard to a minor). As a conservator, you should have power to bring actions on that person's behalf (though, again, you couldn't practice law without a license).