my friend s vet prescribed lasn per day he

I am thinking you mean Lasix 12.5 mg ?? Lasix is a diuretic (people call them "water pills") used for as you say a "leaky heart valve". This is also called mitral regurgitation, or mitral insufficiency. The mitral valve is the valve in the heart that separates the upper and lower chamber. As dogs age, especially Schnauzers, the valve becomes worn out and blood leaks through. Lasix does not help the valve, but what it does is prevent the lungs from filling with fluid due to the leaky valve. It helps the kidneys excrete more fluid from the body, and therefore it doesn't build up in places like the lungs. (which is called pulmonary edema, and is a very common sequela to a leaky heart valve.)

Diabetes isn't involved in this, although Schnauzers are also prone to diabetes. If there is diabetes, it is a completely separate problem from the heart problem.

I hope I've answered the question. If I was wrong about the medication-please let me know. If the label was hand written, you may have read it wrong?