People often use “they” as a way of avoiding saying “he/she”. I tend to duck the issue by using the plural:
If a lawyer wants to take annual leave, he/she must complete a form. [cumbersome]
If a lawyer wants to take annual leave, they must complete a form. [simpler]
Lawyers wanting to take annual leave must complete a form. [I prefer this.]
Lorraine Berry argues that using “they” will help counter sexism:
https://www.theguardian.com/books/booksblog/2016/may/05/they-the-singular-pronoun-that-could-solve-sexism-in-english?CMP=Share_iOSApp_Other