Someone who may or may not be an enemy at a different point in time.
So a friend is someone who more or less aligns with you at that specific moment.

((I would say this excludes true (best) friendship, which is based on decades of sustain. Even if your views don't align mostly, you can still get along thanks to a solid foundation and memories. I'm happy I'm blessed with a few of those))

That said, the weaker minded a friend is, or the more desperate his situation is, the easier he would turn on you.