I had my gallbladder removed in 2000. I was very lucky in that I had had no pain, just discomfort and I was sure that something was wrong. I went to A & E where I had a scan and was operated on almost immediately. The gall bladder was the size of a tennis ball and the stones (we were given them in a bag to keep after the event) were like several bits of gravel. The lady in the next bed had the equivalent of 3 peas.
It wasn't keyhole surgery, but I know my innards are a mess and I told them it wouldn't be any good trying. They didn't believe me and prepared me for keyhole surgery, which meant I had that awful pain in my shoulder for days afterwards, but actually had it removed conventionally and I also have a scar under my right breast which is about 6" long. (No - I've just measured it - it's 17 cm)
Enough of me - this was just the worst case, and will probably not happen to your SiL.
It is advisable - nay, imperative - to keep off fats after the op. They will give your system far too much to do - don't forget, you are no longer producing the juices which break down the fats. If he can get into the habit of a low-fat diet, he will reap the benefits: no pain involved in digesting, less problems such as wind and indigestion, and, of course, a welcome loss of weight.