Assuming the documents are being opened on your daughter's computer rather than contents being pasted, and that both are opening in the desktop application (i.e. Word 2007) and not Word online, I have no explanation.
What tells you that the styles are missing?


What happens if she sends the document back to you? Do your styles reappear?

Any chance you could attach a couple of sample documents, one from your computer and the second being the same one after it has been opened on your daughter's computer? Attach a Sample File Demonstrating the Problem - Here's How