do someone a power of good
and do someone a heap of goodRur. to be very good for someone. You should take a vacation. It'd do you a power of good. Just hearing your voice does me a heap of good.
do a power of good, to
To effect a great improvement. This somewhat unsophisticated turn of phrase uses “power” in the sense of “much” or “a great deal.” For example, “Those vitamins will do her a power of good.”