The presumption here is that you can correctly identify a startup idea that sucks. Here's a quick experiment you can do right now to see if that is true. Try to think of at least one startup that you cannot understand why it is getting traction, press, funding etc. I'm willing to bet you can think of several (I can even think of a couple companies that have actually gone IPO that I still think are dumb ideas).

The fact that you think an idea is dumb is irrelevant. The only opinion that matters are those of the customers. Instead of telling your friend that their idea sucks, ask them what their (potential) customers think of the idea. If she doesn't know, and you really feel like being helpful, push her to get meaningful feedback from a couple customers.

Having a real life customer tell a would-be founder that their idea sucks is so much more impactful than the same feedback from a random friend. Plus, there is the added benefit of saving your relationship.