What CMB said
If they are both boys or both girls it would be fine
However if you keep a pair together there are some important things to know about setting up a cage:
1) It needs to be bigger than for a single dwarf hamster.
2) You need at least two of everything - houses, wheels, food bowls, water bottles.
3) No platforms or levels at all (one can get territorial over it then they can fight).
4) No tubes or tunnels other than large ones - eg rat sized - or one can guard or block the entrance (territorial again leading to fighting).
5) Houses need to have multiple entrances (at least 2 or 3) so one can't block the other in or out.
They may just sleep together in one house, but you still need another one so they can both have their own space if needed.
Wheels - it can be good if one of the wheels is a syrian sized wheel and deep enough for them both to run in together easily. Some dwarf owners have 3 wheels. A syrian wheel, dwarf wheel and a flying saucer would be good. They always have a choice then without being in the same wheel if they don't want.
So if your friend wants to keep them together it's important to have this set up to help them stay together without falling out. Fights can be nasty and to the death and very sudden so the slightest sign of squabbling and they need separating. But with the right set up they have a chance of living their lives out together.
I have read it even goes so far as not taking one out of the cage at a time - taking both out together - otherwise, in even the short time one is out, the other can get territorial over the cage.
Your friend would probably need between 80 to 100cm cage or tank for a pair of dwarf hamsters. 90cm is a good size. Enough space to fit the extra houses and wheels in and still enough floorspace left.
Did she have a cage in mind? Zooplus have a good range.