Good chance he was out on a splav, and going from one to another. It’s easy to get separated, but there are far worse cities than Belgrade to become separated in. It’s a very safe city - I’ve been quite comfortable as a woman walking through Kalamegdan at 2am. The Balkans has a bad reputation, but with the possible exception of Athens the violent crime rates in major cities are very low when compared to America or Western Europe. There really aren’t ‘bad’ areas in the same way.
There’s no need to rely on WiFi. He can use a virtual sim, or just pick up a cheap Serbian one from a shop or one of the street kiosks.
Also, the majority of people in Belgrade will know English.
Tell him to download the Beograd Plus app, because it will give him all the information on public transport he may need (as another poster said, public transport is free). Also, the Car Go and Yandex Go apps are basically Serbian versions of Uber. Obviously he would need to keep his phone on him, and functional, for these.
Tell him not to get tourist-trapped in Skadarlija. It’s fun to visit and an experience, but the restaurants aren’t great and more expensive than they should be for the quality.
The best food in Serbia is in Niš, it’s about 3 hours away by bus, going south. I’d recommend doing a day or two there. It’s very different to Beograd, much more Ottoman. Conversely, north of Beograd is Novi Sad, which is all Austria-Hungary/Hapsburg.