Hmmm... kidney issues? I'd be concerned about that. If her creatinine clearance is normal, I guess she is okay. But that dreaded acidosis can be a side effect of metformin. Its cousin Phenformin was taken off the market world wide because of acidosis side effects. Acidosis causes mental confusion and other symptoms before it gets mega dangerous.

http://www.medscape.com/viewarticle/714920

http://en.wikipedia.org/wiki/Metformin

There is another treatment for PCOS which is quite popular especially in women trying to get pregnant:

http://www.chiralbalance.com/

I took this for over a year, and found it worked on androgen elevation nicely, but it did not stop my insulin resistance much.
For a normal weight woman like you describe, this OTC supplement may be more gentle for the PCOS part. It is expensive however. Chiral inositol is related to myo inositol but seems more effective for diabetics in general. No one understands exactly how it works, but studies have shown it is comparable to metformin for many women with PCOS. It is much gentler.