Some names just have no translate!! Yes, we spell it like Джастин, but I think it's bad idea to tell your friend,that his name is Ustin
Imagine, that some day he will go to Russia, and russian girl will ask him
-What is your name?
And he will answer "Устин"!!!!!!
She'll be in faint, I can compare it only with Romeo or....I can't even compare)))
If your friend'd come to Russia and it will be strange for me to call him Justin, I'll call him may be JEnya (Женя, full name - ЕвгЕний)...But of course not Ustin, because it's a VERY old name.