In many ways bikes are like autos: the difference at a given price point is as much about preference as anything else. If you don't already have a go to LBS, do some shopping for a bike and a shop. Ask the person you are borrowing a bike from.
EDIT: Sorry, your title was advice on a used bike so a LBS probably isn't your main source, but the rest still applies. Look for damage to the working parts (derailers, brakes...).