Do somebody a power of good
Definition 1 of 1
Pro Tip 1/3Positive Change
Phrase implies a significant and positive change in someone's condition or mood. 
The good news from home will do him a power of good.
Pro Tip 2/3Colloquial Use
Used in informal conversations among friends or family, not in formal writing or speeches. 
You look stressed; some time off would do you a power of good.
Pro Tip 3/3Emotional Support
Often refers to emotional or psychological benefit, not just physical. 
Talking to a counselor will do her a power of good.