Are these several different computers in the same house? Two or more computers can be hooked up in the same household and share the same internet which means same IP. Now if you tried a computer in a house that is far away from yours and has a another IP (You can check at whatsmyIP.org) and you confirm they are different, but the server is saying they are the same, this may not be a problem with her IP rather than the server website experiencing an issue.
If it a laptop she tried to sign up at your place with first, then went back to her place and tried there, the website signup may be confused to which internet IP the laptop has and think that it is still the same. Again, you said you tried on different pcs, so this may not be the case.
If all else fails and she cannot signup with that IP address, she may have to change it. You can learn about that..
here. Just tell her to follow the instructions and hopefully that will help with the problem.
If -that- doesn't work, maybe Kretol can work around the system and allow her to be able to signup using some website admin powers by seeing who else has the same IP as hers and remove it from the list somehow.
~ Also!! Forgot to mention!! There is a second thread on the technical support forums with Technomancer having a similar problem! It may very well be a server bug that is returning the wrong error message back to her. You'll want to check with Kretol on that.