Minecraft won't let me log in due to my user being invalid! [duplicate]

Let her login onto Mojang website first, using email of the registration. If THIS works, there is nothing really bad with that account. If that does not work, use some steps to retrieve passwords & safety questions etc to get back the account.

If that works and she used SAME credentials in ORIGINAL Mojang launcher (from Minecraft website ) but the game still does not let her in - probably her IP got blocked up for trying too many times , so no login will be succesfull from that IP for some time. Then she has to try it from other IP or wait at least 24 hours to unblock it. Mojang servers have some security thingies like this to defend against people trying to steal passwords.