Will losing weight help fertility with some one who have poly cystic ovary syndrome?

Most likely. A good strategy for increased fertility in pcos patients with excess body weight is to lose a little bit of that weight while possibly simultaneously undergoing medical strategies. It's possible to get pregnant doing just one or just the other, but combined is usually better.
If overweight, yes. Assuming anovulation (having irregular menstrual cycles) due to pcos is the cause for infertility, losing as little as 5% of one's body weight in the presence of obesity may trigger regular ovulatory cycles, thus improving one's chances of conceiving.