what stock to advise them to have.

I would say start with some zebra danios....or neon tetras... they are very easy to care for and very hardy.

Fancy Guppies are great for beginners. They are pretty and they have fry all the time which is always fun. Neons complement them well.

3 or 4 Black Skirt Tetras and six or so Neons or cardinal Tetras!

Tetras ROCK!

There's no way tetrafreak's suggestion would work for them longterm, IMO. Not only do blackskirt tetras get to be about 2.75" when fully grown (my parents in law have some old ones that large), but they are schooling fish which should be kept in groups larger than a 10g tank allows.

And I have to say also that neons are remarkedly unhardy fish; they are very prone to dying off, especially when first purchased. Maybe glowlights would work instead....but if they went the route of small tetras, I would do ONLY tetras and nothing more, and no more than 5 or 6. Even then, 10gs are best for solitary fish or shimp and frogs, IMO.

Sensible choices (assuming a well maintenanced tank) for beginners include:
A betta
A dwarf gourami
Dwarf corydoras catfish
African Dwarf Frogs (there's an article in my signature on them)
Some of the smaller killifish
Guppies (assuming all male or all female unless they have a plan for any babies...which will be numerous)

IMO, Danios require more space than a 10g can give and they don't look that great in a tank because their constant movement makes it look "busy"

I have to agree these have to be (in the beginners eyes of course I'm saying this becuase I love my cockatoos more then either my neons or guppies) beutiful fish. And they are pretty hardy.

I'll second the thought of the guppies (thought not all male/all female). Have just enough fake plants to allow some of the fry to hide...but most will get eaten by the parents/other guppies. This will help prevent overstocking.

Also add a few khuli's for a clean-up crew. They're really interesting fish to watch, and I'm sure your girlfriend's sister will love them.

I'm planning a similar setup right now for my 12g, though I might go with another type of small livebearer depending on what I see at the LFS when I pick the fish up after the cycle finishes (estimating next week now).

Shellies!

Lamprologus multifasciatus/Neolamprologus multifasciatus

Shellies go great in a 10 gallon and are a most unusual fish.

http://www.thetropicaltank.co.uk/Fishindx/neolamp-multi.htm

I want some!

Roan

Dwarf frogs are really cool, but they can be really delicate to water conditions if you're not careful. I say to the new owners of the 10g that the frogs should maybe wait a few weeks until the tank is established. Also, try to PICK the ones you get out of the tank at the store. These little guys can come home with all sorts of fatal diseases and nothing's sadder than flushing a one. :sad:

But they are very cool to watch and so neat to just look at while they sit there! Make sure to give them a few hiding places (preferably ones you can see in to!

Best of luck!

Jade