I'd PM them about it. If they don't respond in 3-7 days or something, then I'd start looking for another weapon and make an excuse for why the previous one left, PM-ing the other person what you've decided on incase they ever come back. Maybe the characters got into an argument of some kind and just couldn't work together? Or maybe one of them was called off to an academy in a different country or something? Or even just keep from specifically saying what happened to them and leave it up in the air.