I'm wondering if yōsei means someone's name or "fairly tail", but it's not a main topic in thi situation^^
yōsei no tomo... correct
yōsei no tomodachi...correct
yōsei no yūjin...correct
yōsei yūjin...wrong
When you say "a friend of someone", you can say normaly " someone no tomodachi " in Japanese.
For example, "I'm a friend of Tom" means "私はトムさんの友達です (Watashi ha Tom-san no tomodachi desu)”
"tomodachi" is the most common word which means friend. "tomo" and "yujin" also mean friend. And I think "yujin" is more formal than "Tomodachi".
If I introduce myself to elder people, I'll say "Watashi ha Tom-san no yujin desu".
About "tomo", we rarely use this word as a meaning of friend.
Hope this will help ;)