I am appalled that, friend or not, the boss thought it fit to tip you the wink like this. It's a breach of confidence and very unprofessional. Even if you were part of the team where your friend is working, this should be addressed direct with the person concerned. This is the sort of stuff which at my place of work, would come up in the annual appraisal, and if people are not pulling their weight, then it's reflected in their objectives for the coming year. This is disgraceful. Poor performance should be dealt with - in our place - with a verbal warning then a written warning, before further steps are taken which can include retraining or capability measures. How do you think your friend would receive it, if you told them that you'd had it on the QT that they should be watching their back?