The groupthink and cargocult programming is strong in this thread. Your architect friend is both right (probably for the wrong reasons) and wrong at the same time.
that it is not as flexible as WebForms
This is just silly. You can do anything with anything. They are both incredibly flexible. In terms of flexiblity MVC is likely the clear winner here as you can easily achieve Aspect Orientated Programming (AOP) using ActionFilters. Another reason MVC is likely the winner here is that dependency injection is thought of in MVC. You can have inversion of control and dependency injection in WebForms but it requires complex implementations involving the Model-View-Presenter pattern.
MVC app will be slower than a web forms app.
This is invalid to claim, as written. Any application can be written slower comparatively as it's a complex process involving many aspects to reach an end product. However, in terms of raw speed. Webforms is substantially faster. https://stackoverflow.com/a/20253243/37055
it is lighter weight because there is no page life cycle, viewstate, etc.. It should thus load faster at the very least
This is also an invalid statement to make. Page life cycle is irrelevant in all aspects because there are corollary life cycles in MVC in regards to controllers and action filters. View state is interesting... if you choose to stuff 100s and 1000s of kilobytes of data into view state requiring every postback to the server to have 1MB-5MB request, yes it will obviously be faster to do nearly anything differently. This isn't webforms fault, nevertheless webforms allows you to fall into the pit of failure very easily with viewstate.
is ASP.NET MVC better for large scale apps than ASP.NET WebForms?
No. Yet the answer to this question "is ASP.NET WebForms for large scale apps than ASP.NET WebForms ?" The answer is also No. The answer is no, because the answer is always it depends. Every framework has pros/cons and you need to measure those, there is not definitive answers.
If you're building a content driven site who's job is to have the fastest possible page load times such as www.microsoft.com then you might very well choose webforms.
which would you choose to use for a large scale enterprise application?
First, you very likely don't have this problem. You would not be in a position to ask this question if you were truly responsible to architect a large scale enterprise application. (or the hiring process didn't actually require large scale development experience).
In terms of a large scale application the framework you choose is almost meaningless. Large scale applications are built on queuing. They will leverage tools such as MSMQ either directly or through a servicebus such as: Mass Transit, Azure Service Bus, or NServiceBus. Only with queuing can you reach the scale to handle millions of requests the way Amazon, Ebay and every other major player does.
