We usually don't use the future progressive (or any progressive) with a verb that doesn't have duration. When you meet your friend in the cafe, the meeting is quick, you say hello and the act of meeting is finished. Then you sit down, order drinks, and start talking. "I'm going to meet my friend" is better for this sentence because it's a plan you made before talking to the person who asked "Are you busy this evening?" Here, the future progressive works: "We'll be working there until 8:00, so if you are in the neighborhood, stop by and say hello." The act of working will last for some amount time.