We say "제 친구"(honorific) or "내 친구"(casual) referring to "my friend". About "his/her friend", there're some words used differently in different situations. If 'he' or 'she' is much older than us, we use the honorific. For example, "교수님 친구 분" or "사장님 친구 분" or "엄마 친구 분". If 'he' or 'she' is your older friend, you say 철수(name) 형 친구 or 영희 누나 친구. If 'he' or 'she' is your younger friend, you say 철수 친구, 영희 친구 or 걔 친구, 쟤 친구, 얘 친구. 걔 = 그 애(short for 그 아이), 쟤 = 저 애(저 아이), 얘(이 아이). 그(의)/그녀(의) 친구 is used mostly in writing, you'll see these only in books.

Hailey did great job for explanation. so actually dont need to do again. my friend=내 친구=나의 벗 his friend=그의 친구=그의 벗 her friend=그녀의 친구=그녀의 벗 pur korean expression for friend is 벗(or 동무). but mostly we use 친구 for that. and 벗(동무) can be used for written Korean, for novel or poem and so on. I hope it could be a lil help for u. have a great evening. :)