The best feedback you can give someone on their work is the simplest.
First, depending on the medium, truly listen and take in (read/watch/listen) what they’ve chosen to share with you. You taking the time is a gift.
Second, find out what they’re going for. This should have nothing to do with your taste and preference. It’s all about them and what they want. Ask a few questions in this direction. You might find that they’re not exactly sure, with which case this inquiry will help them clarify.
Finally, if there were moments you really loved, point those out. And if there were moments where your attention or focus drifted or you felt lost, point those out too. The contrast of moments will prove instructive.
And that’s it. You’ve now given plenty.
Unless they specifically ask for you to help brainstorm possible solutions, don’t offer. That won’t help. Trust that they will solve it all by themselves. And they will.