It was for 50-50, not some bullshit. Start by asking how they envision their half of the remaining work. Listen to what they have to say. Probably they haven't really thought about it. In that case, they will have a more realistic understanding of the context and what the offer looks like to you. That doesn't mean they will like what they see, just that they will know what is rational.
On the other hand, your friend may have thought the idea through or be willing to do so and if so who knows where things could go. There's a chance that the magic is there.
But I think the best is that you say: I think this is such a good idea, you should keep 100%. Let me teach you to programm for free. We'll start w/ some basic apps. Here is a good book you can refer and a tutorial site. I'll meet you Saturday's to work w/ you."
On the third hand, finding a student who actually stuck with it through a year of Saturdays would be a wonderful experience.
"Ideas are just a multiplier of execution" http://sivers.org/multiply
A variety of: "Ideas are nothing, implementation and people everything."
If you think it is a genuinely good idea then say. Maybe offer to be his goto. So when he has issues, or just needs to talk about something related to the project he can come to you.