"They would be friends."
Translation:Ellos serían amigos.
Ser is always used with amigos. Forget the temporary and permanent rule and just pay attention to actual usage of Spanish phrases. Listen to and read lots of Spanish and things like this will just sound or feel right or wrong to you.
This use of would, in English, is not just for the conditional. This used of would is also know as "the future in the past" Sounds confusing, but here is an example.
John and Henry met for the first time in 1985. They would be friends for life. Meaning, there were going to become friends and this friendship would ensure their entire lifetimes.
The same thing can be done in Spanish with iba + a+ infintive.
Ellos iban + ser + amigos de por vida.