She's got a problem. If she has bad US credit and she wants to be able to be reached by dialing a local US number (assuming that it's a US GSM carrier's number.) To be able to call a US number and be reached in Europe requires international roaming. International roaming in western Europe is 99=A2 per minute or more (depending on whether a major operator such as T-Mobile or cingular is used.) The carriers won't even allow you to have international roaming unless you have really good credit since the ability is there to rack up really big international roaming bills.
You'd still pay a huge amount of money with roaming rates as high if not higher than the other way around.
International roaming does not exist on prepaid in the US. Even not all prepaids in Europe offer international roaming and the ones that do it's quite expensive.
It does not exist. Roaming in another country is expensive no matter what you do.