If she is inserting the signature into the original message it's probably only setup to spellcheck the new message on the reply.
Go into Tools--->Options and click on the Spelling tab. Uncheck the box that says "Ignore original message text in reply or forward".
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\"All over the place, from the popular culture to the propaganda system, there is constant pressure to make people feel that they are helpless, that the only role they can have is to ratify decisions and to consume.\" -Noam Chomsky