If the cards are anything like the prepaid cards we have in Europe (Sweden more specifically), they're prepaid with a given amount of credit, and when that is consumed you can reload it by entering codes that you get either off a scratch card that you buy, or from a machine that prints it out after you feed it, or its owner, money or your CC. Here, these cards can be purchased most everywhere.

If she passes through Nairobi I'd think it'd be easy to find a place to buy one - and a couple of "recharges" if she's fond of phoning home...

/Michael