Sooner or later it's bound to happen. Someone discovers that you're a writer and presents you with your manuscript asking for your opinion. It would be easy to see this as an imposition. After all, you have your own work to get on with and someone else's project is yet another reason to procrastinate. Instead, take it as the compliment it is.
How you tackle the task you've been given depends on how experienced the writer is, how long or complicated the manuscript is and how well you know the writer. Be aware that even when someone has asked for honest criticism, no one likes having their work pulled to pieces and you wouldn't want to ruin a beautiful friendship! That isn't to say that you should tell them it's wonderful when it isn't. That helps no one. Simply make sure your comments are constructively put and try to team anything that might seem negative with a positive. For example, suggesting that a sentence should be cut sounds as if you think it was rubbish, rather than unnecessary. Try using 'condensed' instead, which encourages the writer to analyse the piece for themselves. If the piece fizzles out but the opening grabs your attention, suggest that the writer tries to rewrite the ending to have the same impact. 
Emphasise that your opinion is just that and that someone else may feel quite differently. In fact, suggest that the writer joins a writers' group or class (either online or local) for feedback and moral support. Remember how it felt when you were starting out, and be careful–you are being entrusted with someone else's dream.