Massively risky. It always is when your proposition is 100% reliant on someone else's with no promises or contracts. What happens if Stripe suddenly say no (a la Padmapper), or they make their own? Just saying.
I didn't get the impression he's trying to make a living off this or anything. I just thought he decided to make something cool based on the Stripe API, and if they make their own, all the better. Whatever happened to making things because they were fun instead of because they had a good value proposition?
If Stripe decided to make their own iPad app, I would be fine with that. It's not going to put me out of business. In fact, I could save them the hassle of having to build their own... :)
I don't think I'd characterize it as "massively risky" - it took him about a month to develop.