They seem natural until she is flat on her back and not holding onto them. Then they seem to not lay a flat as they should. Many times breasts that large sag or flatten quickly, so a small-ish implant could be used to 'fill out' her boobs to make them look better.
Most boob jobs I've seen (99%) look fake to me, but the best looking ones looked better as there was significant breast tissue to begin with.