That's a helpful metaphor. What I'm looking for is a framework that can be used for rapid prototyping, and it sounds like rails fits the bill.
Sinatra is more of a "micro-framework" and puts you closer to the http method.
Basically, the api mimics http more closely, you literally use get, put, post, and delete methods to define your routes and actions.
In Sinatra, you define your routes in the same place where you program your Resource's functionality, so less mapping is required. In Rails, you have a separate controller and route files.
The model layers between Sinatra and Rails are very similar. For example, you can use ActiveRecord in Sinatra.
Not having to write migrations (as you would with ActiveRecord + Mysql) is huge.
Also, your models can be more flexible. You can add keys to your document as needed. With Mysql, you need to have a migration to add the column.
Second, I'm tired of all the NoSQL hype. I've worked on a project with MongoDB and MongoMapper, and the combination caused more problems than it solved. If you want cutting edge and all that comes with that, go with Mongo. Otherwise, if you'd prefer to focus on problems in the domain you're trying to solve and not the technology you're trying to solve it with, go with something relational and tried and true (PostgreSQL, if I were deciding).
When all you have is a hammer...
As for migrations, I don't think he's gonna be dealing with anything too complicated schema-wise. Django South will write your migrations for you. Maybe Rails has something like that? But honestly, I'd recommend using SQLite and just starting a new file whenever he really felt the need to alter the schema.
Of course it depends on the problem, but working with hashes and arrays is simpler than the relational model, no?
I do concede the vast swaths of docs for AR, however AR is pretty complicated and issues arise that aren't well documented.
With Sinatra, I'm finding it to be less work.
I would probably stick with relational tech if starting off with Rails, but keep your eye open toward alternative store, such as document databases and key/value stores.