I'm a backend team lead for a travel company. To render that list of flights and hotels to the Android app it's not a database lookup. It's hundreds of soap and rest calls stitching together airline and hotel data from multiple platforms. This isn't something I can do on the front end. And even if I could I wouldn't want to replicate this logic between iOS, Android, web and our internal support portals.
I suppose it is fine to have business logic in a client, but replicating that for every platform seems silly and a simple way to acquire huge amounts of technical debt and complex role out strategies. He says about being the only backend guy to ensure not much code gets written there. I feel like this is not a person I want to work for. Not even seeing the problem before telling me the solution.
Funnily enough that actually was the original approach, and it worked "OK" up to a point, except in Internet Explorer 11, which couldn't handle the memory consumption, and honestly it was dog slow everywhere else: both loading the data and doing the calculation. And of course this approach is a disaster on mobile.
Even if we could get calculation performance on the client that is better than the sum of time spent on the server plus latency, and maybe we could with WebAssembly or WebGL hacks, except that we'd probably have at least some incompatibilities to deal with (hey, starter for 10, IE11 again - who knew?), there's still raw data transfer to consider.
So now most of the smarts are in the back-end, and they'll stay there for the foreseeable future.