I'd recommend Godot. Unity is also a valid choice, but networking is in a weird state right now with the official APIs, so he'd have to use third-party solutio

I'd recommend Godot. Unity is also a valid choice, but networking is in a weird state right now with the official APIs, so he'd have to use third-party solutions that integrate with Unity (such as Photon, MLAPI or Mirror). It's not the biggest obstacle, but still an annoyance.