Hawk Diesel
Adventurer
@ad_hoc has the right idea. Part of it is contextual. If you have a person who is new to DM'ing, you need to inspire confidence first and foremost. If you are too critical, that will discourage this person from wanting to DM. Being a DM, just like any skill, requires practice, and no one is good right from the get go. It takes time to nurture that skill and learn how to juggle interpreting rules, creating fun challenges, sharing the spotlight, valuing player input, and creating and interesting narrative while also managing all the enemies and NPCs.
Feedback is important, but if you are providing feedback, you need a few things. You need to find examples of things the DM did very well. Then you need to narrow down the critique to just one or maybe two really important pieces of feedback. Then when you communicate them, frame them in a way that focuses not on how the DM did things wrong, but rather how you might do it differently or how it could be done to make it more fun. This way it is not a knock on the DM, but rather advice on how a slight tweak can enhance the game. It's also important to use concrete examples whenever possible.
Feedback is important, but if you are providing feedback, you need a few things. You need to find examples of things the DM did very well. Then you need to narrow down the critique to just one or maybe two really important pieces of feedback. Then when you communicate them, frame them in a way that focuses not on how the DM did things wrong, but rather how you might do it differently or how it could be done to make it more fun. This way it is not a knock on the DM, but rather advice on how a slight tweak can enhance the game. It's also important to use concrete examples whenever possible.