"When he agrees to be Papp's reference, there is tacit agreement that the reference will be favourable."

The employee may believe that, but it's just not true in law or in reality.  Very few "tacit agreements" are given legal weight.  

If the employee wants to know what a potential reference will say, he or she should ask.  It might also be wise to ask the reference to agree to notify him or her if new information comes up that causes the reference to question the good opinion.