Let's see, the cages appropriate for Roborovskis Dwarf Hamsters are any cages that are at least 7 mm bar spacing, cages such as ... The Barney Hamster Cage, Savic Mickey XL, Alexander Hamster Cage, Skyline Cages pretty much, the Ferplast Mini Duna seems small I suggest getting the Ferplast Duna Multy if your boyfriend still wants a mini-duna style cage.
There is also the Skyline Kerry if you're interested in Terrarium cages. I don't think it's true that large cages make them skittish.
Anyways, there's also the Ferplast Kios which has 7mm bar spacing I think, since it was made for mouse and dwarf hamsters.
EDIT: Here are the links for the cages.
Barney Pet Cage | Great deals at zooplus! (Barney)
Hamster Cages: great selection at zooplus: Small Pet Terrarium Kerry (Kerry)
Alexander Small Pet Cage | Available at zooplus! (Alexander)
https://www.amazon.co.uk/Savic-Micke...ic+mickey+2+xl
(Savic Mickey 2 XL)
https://www.amazon.co.uk/Ferplast-Du...ast+duna+multy
(Duna Multy)
If you're also interested in this seemingly generic cage but has 7mm bar spacing.
https://www.amazon.co.uk/Mouse-Hamst...ter+cage&psc=1
https://www.amazon.co.uk/Ferplast-Ki.../dp/B00F5ZYQNW (Ferplast Kios)