the plumber – Dan Likav Zechut in this case would be either 1) they checked into it and are careful only to drink what is actually kosher or 2) although they may actually be drinking non-kosher coffee it is being done b’shogeig (without intent to sin) based on misinformation.
We do not have to go so far as to make up unrealistic scenarios just to place people in good light. There are enough “realistic” possibilities to believe in to place others in good light. This is what is truly meant by Dan Likav Zechus.
I heard a story once that a Mashgiach of a renown Yeshivah was informed that a bachur was smoking in his room on shabbos. He went down to the room and saw that the room was full of smoke. Not coming to conclusions he asked the bachur for an explanation why the room was full of smoke. The bachur explained that he had a strong urge to smoke and so he smoked up until he needed to be mikabel Shabbos and then put out the cigarette. Being a smoker he didn’t feel the need to open the windows and that is why there was still smoke in the room.
In this story we see that the other student who informed on him jumped to conclusions when he saw the smoke. All he had to do was think realistically and he could have easily came to the correct conclusion.