Hamsters generally do not get along together. They're ok when they're still young and might be ok if they're litter mates, but generally they don't like each other.
Hamsters breed like crazy, I had dwarf hamsters and you can NOT tell them apart. Larger ones are a bit easier. But it's true, they do eat their young, especially if there's too many in a cage.
Cages are a pain to clean, especially tubes. They come apart, but it's annoying to take it all apart and put it back together again. You're talking an hour or so to do this. Granted I'd do a major cleaning like once a week, but freshen every few days.
They can bite. My first two hamsters were the sweetest little things. My first dwarf, little buddy, I could wake him in the middle of the day and he'd be just fine for a bit of loving. Teddy on the other hand didn't like to be woken up in the middle of the day, he was alright if it was evening. But I had several that would bite if you put your hand to them, especially coming down on top of them, they feel threatened.
They're pretty good if you travel, you really only need to clean the cage about once a week. And they store food in hiding spots, so you don't have to worry about over feeding them. You'd just have to worry about them running out of water.
They are very good at escaping, and knawing through anything that looks amusing. So you might end up with quite a bit of damage before you find them. You generally have to find them at night when they're normally "awake".
The cages are really expensive. And even if the two you get, get along together. I would still suggest connecting two decent size cages together to give them personal space. I had a critter trail 3, critter trail 1, and a critter trail mini. And the 3 is like 50$ or more. I mean it's more of a one time cost, but I had mine even chew through the tube connecters (at least they sell these individually for replacements). And the tubes are expensive too. Pine shavings aren't that bad, I could get a big bail of it at the pet store for like 10$ and it would last for ever.
http://www.petsupermarket.com/search.aspx?Search_Query=crittertrail - here's a general idea of prices.
Tell her she can have them if she can buy the cages. Then go to the store and let her start picking out the cages and accessories. Write down everything she "wants" with the price, then show her how much it is. And the food isn't all that cheap. And since they do a lot of "storing" food, you wind up throwing a lot out when you clean the cages.
Having a dog was a lot cheaper.. even if you include the vet bills.