I don't have a lot of developer/programmer friends most of my friends are mostly normal people. So something like this could happen. Sometimes as a Developer, you got that one or two friends telling you something they need.
Here the common pickup line:
"Hey, Could you build this app? I may pay you..."
"Hey, I don't have enough money, but I can sell this app for you"
"This app can you make it? But add these options..."
Although I'm busy working for somebody else I think I've still had a chance to make those friend idea. I'm serious here I created a Trello board for it.
If you wonder what is that label for here it is :
Your friend's idea is possible to make. That's not an app to hack Facebook or Instagram account (We all been there "Could you hack his Instagram account he seems cheating on me").
I always, always, and always say this to my friends, "I maybe can make those apps if you can explain it how it works or how to do it". So when they ask "Can you make an app that could hack some Facebook account?" I'll reply "Tell me how to hack Facebook first, then maybe we could make it"
Really, the app is worth to make. It is something outside your comfort zone, something challenging!
You may don't want to work for free, maybe your friends want to pay you, or those ideas can bee a product or startup!
You already take that Idea seriously! Maybe that's just a silly app, or something not that big, but you already got time and got some research on the app so you create a board and tell your friend "Hey, that app you tell me, I'm on my way to make it, maybe it took a long time, maybe not :)"
Just take a note cause every Idea worth to hear.