Just a situation ive had stuck in my head a few years... Think a Ferengi contract lawyer hired by an alien visitor would be capable of resisting the temptation to trip him up in red tape, to encourage other wealthy aliens to come his way? As its sometimes hard to see if the ferengi are aware of how their radical mercantilism is offensive to some. just a thought.
It would depend on the individual and situation. Some Ferengi are more ethical than others, so some will and some won't. Also on the profits. If the profits for not tripping his client up is more than doing it than a smart Ferengi would chase the path of greater profit. Or course if the Ferengi is desperate they are more likely to take the immediate profit.
You don't encourage wealthy aliens to hire you by openly backstabbing somebody else who hired you. In The Nagus, Zek shows awareness that their opportunity for profit in the AQ has been curtailed by their reputation for dishonesty. I think they're very aware that their attitudes are offensive to aliens. The Ferengi think of themselves as rational businessmen but they are not all that rational, opting for short term rewards with higher risk over long term rewards with less risk. Ferengi are naturally risk-seeking. They're terrified when confronted directly with physical violence, but they will run straight into situations that risk them physical harm if it means a greater chance at profit. They might behave that way and milk their clients for extra hours just because it's their natural inclination. Or if it's not that, it's the culture. Just like human men have to prove themselves manly by taking stupid physical risks, ferengi men have to prove themselves manly by taking stupid financial risks. Screwing aliens over is the Ferengi version of cliff diving.
I always thought that it's not in the Ferengi's best interests to cheat people. Word will get out, if they do that, and they'll lose customers. Keeping a good customer is a cornerstone of Ferengi business practice, and so therefore anything that will openly drive customers away is not something the Ferengi would do.