Не говори с косым о кривом

See В доме повешенного не говорят о веревке (В)

Var.: С кривым о косом не беседуют

Cf: It's ill halting before a cripple (Am.). Name not a halter (a rope) in his house that hanged himself (that was hanged) (Br.). Never talk of rope in the house of a man who has been hanged (Am.)