Next friend, in British law and American law, is the phrase used for a person who represents in an action another person who is under disability or otherwise unable to maintain a suit on their own behalf as a result of their circumstances, who does not have a legal guardian.
This disability often arises from infancy, mental incapacity, or lack of access to counsel, consequently every application to the court on behalf of an infant or a lunatic or a person detained without access to an attorney, who does not have a legal guardian or someone authorized to act on their behalf with a power of attorney, must be made through a next friend (prochein amie, proximus amicus).
Prior to the Married Women's Property Act 1882 in British law (and similar acts in the same time period in American law) it was also usual for a married woman to sue by a next friend, but that act, allowing a married woman to sue in all respects as a feme sole, has rendered a next friend unnecessary in the case of married women.
When a relative who is next of kin acts as a "next friend" for a person, that person is sometimes instead referred to as the natural guardian of the person.
Historically, in the case of an infant, the father is prima facie the proper person to act as next friend; in the father's absence the testamentary guardian if any; but any person not under disability may act as next friend so long as he has no interest in the action averse to that of the infant. A married woman could not historically act as next friend, but this practice is no longer current, at least in the United States. An infant frequently defends a suit, not by a next friend, but by a guardian ad litem often appointed by the court with jurisdiction over the case or a court with probate jurisdiction.
In the case of mental incapacity, a conservator, guardian, or committee, but if they have no such representative, or if the committee has some interest adverse to the claimant, they may sue by a next friend.
A next friend has full power over the proceedings in the action as if he were an ordinary plaintiff, until such time as a guardian or guardian ad litem is appointed in the case, but is only entitled to present evidence on the same basis as any other witness.