"My best friend had taken my boyfriend."
Translation:Min bästa vän hade tagit min kille.
I realise this is a late reply, but in case somebody else enters the thread and is wondering: you can also use kille for boyfriend colloquially - but not pojkvän for guy. The latter is, as you say, exclusively for a romantic relationship. Also, the same holds true for tjej / flickvän (girl / girlfriend).