Best advice I can offer, is think of how these fiends are (at the fundamental level).
Ultimately it's up to you, and your setting, but typically full fiends (demons, devils, daemons, etc etc etc) are heartless. They only ever care about themselves, and advancing their own agendas, whatever those agendas happen to be.
The concept of "friend" just doesn't exist for them.
Sure, you can change that for your setting, if you want, but it will take a fair bit of work to ensure verisimilitude can be maintained.
It can be done, sure, but the first thing you'd really want to consider is the
why of it. "Why is this character different than normal fiends?"
I'll use my own setting, for example.
Fiends (true fiends) are monsters.
Fiendlings are the result of tainted human bloodlines. They're mostly human in appearance, and mentality, so while they have a strong leaning towards being evil, heartless, monsters; they don't
have to be
. They are more "human" than "fiend", and thus the human side tends to win out.
Beyond that...
Like
@L1aei already said. Use real human relationships as a reference point, and then filter them through the lens of the character(s) involved. That really is the only "secret" to writing believable relationships.