reduce breast size naturally

There is no way to target which area of the body you lose fat from.

I would reccommend cardio excerice (running, jogging, aerobics etc...) as weight training can build up her pectoral muscles, making her breasts appear larger.

Your breasts are made up of two kinds of fat. The first is the fat that determines your breast size... your hormones decide how much of this there is. The second is excess fat carried by your body. Excercise will help her lose the 2nd kind, but not the 1st. The good news is, that the breasts are often one of the first places you lose fat from (if it's the 2nd type)

She could also purchase minimising bra's to make them appear smaller.