I'm not a huge fan of North African food, so I don't have any good recs -- this said, you can't go more than a block in Brussels without tripping over a Moroccan/Tunisian restaurant specialising in couscous or tajines.
If your friends are interested in expanding their African borders southward, they should wander around the Matonge neighbourhood -- west of Chaussee d'Ixelles, south of Chaussee de Wavre and around the Place St Boniface. Brussels has a huge Congolese/Senegalese population, and this is where a lot of Africans have settled. There are a ton of little restaurants -- one I've been to and quite enjoyed is l'Ile de Goree in Rue St Boniface.
Asian restaurants? If they want Vietnamese, they need to head over to Chaussee de Boondael, near the Universite Libre de Bruxelles. or to the neighbourhood around Place St Gery downtown. Be warned, though -- I find most Asian restaurants bent on serving insipid, dumbed-down-for-Europeans food. A few exceptions:
Rue des Poissoniers (just a couple of blocks from the Bourse and the Grand Place, on the cusp of Brussels' pathetic excuse for a Chinatown)
No atmosphere, but tasty Thai food. Curries seem to vary in quality, from ethereal to mundane, but the som tam is consistently excellent -- and HOT
Rue Francart (again in the Place St Boniface neighbourhood)
Closed Sunday, Monday, Thursday
This tiny, tiny, TINY Japanese noodle shop has people waiting for hours – which can seem like days, if you’re hungry enough – for one of its 12 seats. On offer: gyoza (succulent), several types of miso and ramen soups and my personal nominee for the best pork and rice combination ever, tonkatsu don.
Chaussée de Boondael
We hardly ever go to the actual restaurant to eat, but this is our go-to takeaway Thai – beef with basil is REALLY good – and the curries aren’t bad either. Stay away from the fried spring rolls, though.
Hope this helps.