Coldplay sheet music for Strings?

I would suggest obtaining piano sheet music and just play from that. Often popular music arranged for piano has a single moving b line which can be easily played by your cellist and three-note chords and/or melody and harmony lines that can be played by the violins and viola. Your violist will likely need to be the most flexible in note reading - either treble or b clef. This would be much easier than transcribing by ear. No, you would not be doing anything illegal if you could write the arrangement yourself.