(All tips here are just that...tips. Above all of these is have fun and be yourself. There are no real rules.)
If you want to help a writer you know instead of being one yourself, know one thing: telling a person something sucks doesn't help them and only makes them ignore any help you might try to give later. If a story sucks, help them improve by making suggestions but let them make the final choice on things and support it when you know they want to stick with it. Most writers do it for fun, and you can take that fun away by trying to help if you don't know how to do it.
On the other hand, saying you love everything about a story without explanation as to what it is you loved or why is of little use either. It might make them feel good but there is nothing to use from your feedback. If the author had great use of dialogue, for example, tell them so they know what works.