I would start by trashing her Mail preferences.
Option-click the Go menu to select her Library. Then go to Preferences folder, locate any com.apple.mail-something.plist files. Trash them and restart Mail and see if that helps.
Also, does she use the same Signature file for each message? See if a different Signature makes a difference (even create a new one, if need be).
First Welcome to the Discussions.
These 2 links may or maynot help but I think are well woth having a look.
I would first go to your Balck Apple and make sure there are no Updates available for your Software.
Followed by reading this 2nd link very carefully and see if this may show your issue.