Yes it is common to have just one breast affected by gynecomastia.
He may develop it in his other breast but if he's had it for a few years this is very doubtful.
I do not think that giving somebody a titty twister would call for his breast to enlarge.
If the other breast has a puffy nipple/areola, it should also be considered for gland excision.
Speak with an experienced plastic surgeon who has performed gynecomastia removal several times before with satisfying results.
Gynecomastia is generally considered cosmetic but there's always a chance that the insurance will pay for it if you can prove that it has a detrimental physical / emotional affect on your SIL's life.
Concerning your SIL's hormones, go to
www.aace.com, go to Services on the left, and click Find an Endocrinologist. Enter in your zipcode and it'll pull up a list of all the AACE certified endocrinologists in your area. Find one that specializes in Reproductive Endocrinology and make an appointment with him/her. I would not trust a general practice doctor with this situation, and so the endocrinologists' expertise is required. Blood work should be taken and discussed with the endocrinologist. Perhaps ask one of the endo's nurses over the phone if he/she is very familiar with gynecomastia and its causes (this is what I did).
From what I understand, pathology is usually an additional cost and is not included in your surgery fee. Typically the surgeon will be able to tell you how much is removed and what percentage is gland (for example, I had 1000cc on my left breast and 800cc on my right breast removed, 20-25% of it was gland) but a thorough pathology report will probably cost you extra. Your endocrinologist may request that you get a full pathology report in order to better understand your SIL's situation if he does choose to have surgery.
I do not know the percentage of males who get breast cancer with gyne.
I don't know of any surgeons in your area but I can share my feelings with you. I'm 18 and just had sugery three days ago after just over a month of waiting. My photos will show you just how much of a difference the surgery has made on my chest, and I feel so far that it was well worth it. He should not feel ashamed in getting surgery.