Oh goodie, I love these questions.
I'm going to assume the following is true about you:
1. You can translate someone's desires for a product into a software spec.
2. You can translate a spec into a real, working piece of software by programming it using your favorite languages and tech stack.
3. You can do the above in a reasonable amount of time, reliably and consistently, and follow a project deadline with milestones without too much supervision.
4. Your friend is asking for this because he wants to market it as part of a product or service.
Great! If you want to be fair to him and give him "mate's rates" then charge him $2000 per week.
Why $2000 per week? Here is the logical progression:
1. You have the aforementioned, assumed skills (which I am deducing from you being a developer of 3 - 4 years) that make you very valuable.
2. Your floor for making an iOS app for a client would be $5000 per week. Why? Because it's mostly arbitrary and it's what the market would pay, and I assure you the market would bear someone of your skills being paid this much money.
3. $2000 per week is a vastly discounted rate from the $5000 figure quoted above, which is appropriate for friends. Note that this is appropriate for friends working together as business partners, not for something so insignificant that a friend is doing it pro bono. That you want to be paid at all indicates this situation is not the latter.
When I do technical consulting I always bill weekly. I fundamentally disagree with billing hourly for a variety of reasons, so I think you're right that you should not be billing hourly.
If you don't want to bill weekly you can set an entire project fee. In that case, I would take the above amount and multiply it by the number of weeks you anticipate working.
What I would emphasize to you is that pricing is basically arbitrary. You simply charge the highest amount the market will bear. If this project is worth doing and "serious" enough to warrant you being paid, you might as well be paid generously, even if it is very discounted for a friend.