Agreed. Because that is an existing client whose contact with your tech is on the basis of theirs and your tech's contract with your company..... i have clients every now an then ask my techs "can you do this after hours", if thats the case thats unethical, period. when that happens i no longer do business with that person. ....
However, if a tech finds outside work on his or her own in the same line of work as their day job, and/or if a person independently solicits for said work to be done, I don't necessarily see an ethical or moral problem with that.