11-28 is way too wide a spread for someone who doesn't have hills to ride. Most people are never going to really Use the 11 (as in, spin it to max revs for any period of time) and 34x28 isn't likely to be needed on flat roads even on the worst of day.
For someone who professes to be seeking close rations for flat roads, the 12-25 packs a few more into that spread, while the 11-28 has only about eight.
As for the 50-34 chainrings ... I'd ride them a while and see. He won't really Need the highest gears unless he really plans to compete, but for the cost of swapping ... I don't think he'd gain much.
I run 48-38-28 with a 14-34 on my tourer, which yields a 90-inch top gear. If he runs 46-11 that is 110. Even with no load on flat roads it's okay. If he wants to go racing, he might need the 50-36.
On the other hand, that 36-tooth ring will take him right up to the top third of the 50 ring in terms of gear inches. if he just cruises a lot, he might find himself riding in the small ring most of the time.
I'd say, try it our and see before spending any money, and if you must spend, check the cost between moving to a 12-25 cassette versus buying a new crankset. I bet the cost is about a quarter and the labor similar--and you would get a lot more of those middle gears.