I mean, you'll like it because it's by your friend and everything, and you love your friend so much - but objectively, what they send you is gonna need a lot of work. And they're really going to want to know your opinion, since you're a writer and everything. They're going to tell you they hope it isn't bad. Oh man. What can you do?
In the end, you're going to reply with something like, "I liked it! I really did, because... [insert praise] But here's just a few things you could work on... [insert kindly-worded constructive criticism]." Your friend will take it well, and you'll feel good for making them feel hopeful about their writing.
Okay, I admit, I can't really tell the future. But who knows? Maybe this situation could happen to you. You could handle it in any number of ways aside from the way I foretold. But I'm guessing that you would make a point to be kind to your friend.
Whoaa. Dude, where did all the depressing just come from? Who would ever consider telling those things to one of their closest friends in the world?
... One question: have you ever told any of those things to yourself?
It seems like sometimes as writers, we can be really, really hard - not on our friends, not on our fellow critique group members, but on ourselves. In every stage of creating a book out of nothing (wow!), we tend to be our harshest critics. Of course, there can be moments when you love what you're doing, or at least you have hope. But how often in the past have you gotten stuck in a needlessly harsh, "you suck" state of mind - even though you would never, ever treat a close friend who sent you their writing the same way?
I guess what I'm trying to say is: sometimes, if you can, remember to be a friend to yourself while you're writing. Because you deserve supportive friend-hugs at all times, and you deserve kindness. Even if - especially if - it's coming from yourself.