Anytime a relationship between Leaders and the Subordinates is 'perceived' as being a friendship then it needs to stop. Leaders should always foster a professional environment and working relationships.
I always told my troops that professional and personal are two different and distinct relationships. You have to be able to distinguish between the two and if you can't then the only acceptable relationship is the professional one. I achieved this by always treating my subordinates the same. The same rules, standards and expectations were levied on all my subordinates. We did get together in an 'off duty' setting on numerous occasions and I strived to include all the subordinates in the activity. I maintained a very relaxed and open setting, but kept the overtone of professionalism. If a subordinate while on duty tried to 'invoke the off duty' friendship to get out of something, I swiftly and immediately corrected that individual reminding them that one had nothing to do with the other.