You should always train every part of your body regardless. All your muscles counter-act each other so only half training can cause imbalances in them. What i think your friend is trying to say is that as you start to train, fat on your body (and legs) will lower and you'll gain some muscle which will in terms of size keep them around the same size. You may find they might even get a little bit smaller if you lose some weight. In terms of what sets/reps to use, it shouldnt really matter. Just follow whatever program you are doing. Increase weight when you can complete the sets without failing (progressively overload) and don't worry too much about size changes, you wont get anything too noticeable.