The second paragraph must be formatted to have the proofing language Spanish. This is best done in a Paragraph Style setting. Otherwise, the proofing language setting is on the Review Tab.

If she alternates one paragraph in English an one in Spanish, she can have the Styles set to automatically switch to the other Style when she hits the Enter key (Style for following paragraph).

That will activate Spanish language AutoCorrect.

It may be that on her old computer she had the option turned on to automatically detect language. This setting causes many users a number of problems so I don't recommend doing that, but it could be that is what she was doing. (Word is not very good at deciding which language is appropriate.)

Note, this is just a guess. I've not tried it.