This could have been a tweet but I would like to elaborate over this topic. Here is a fact:
This is explanatory enough but it is not that simple I understand, we say, but they are friends. I'd say but they a going to earn profits out of it. It is as simple as that, some time down the line they are going to earn from what you did for free if they plan on any form of business.
There are always some exceptions to it, like they are not going to get absolutely anything in return or they are doing if for a good cause. I have designed for friends, who now earn from it or not, but they do, I as a designer was just a cheap/free of charge resource in the name of a friend.
When they say they do not have enough budget, ask for something in return, if they have the audacity to ask to get something for their benefit, in my opinion, you should too.