The trustee of a revocable living trust may have much the same authority to deal with someone's finances as the agent does in a durable power of attorney. Even if he has a living trust, however, it's still a good idea for him to execute a durable power of attorney for finances. (He could name the same person to both jobs -- trustee of the living trust and agent in the power of attorney.)

The reason it's wise to have a separate document is that not all his income and assets may wind up in the living trust; if some income or asset comes to him after he's incapacitated or wasn't placed in the trust through some oversight, the trustee would have no authority over it.