I want to draw the attention of members to an important point. If a general surgeon has learnt to do urology surgery by his colleague then can he practice urology?

Is it legal? Because I know many Doctors who are general surgeon (MS) but practically doing every procedure in urology (PCNL, TURP, UR and cases like that) If a general surgeon does urology and some complications occur, then how can he defend himself in the court of law?