To select the library tom be opened in Photos hold down the options key ⌥ while launching Photos. Release the ⌥key, when you see the library chooser dialog , then select the library you want to open.
If your wife does not yet have an own Photos library and wants to see her iCloud Photos Library, she could select to create a new library, when the Library Chooser dialog appears. Then she should make this library her System Photos lIbrary in the Photos > Preferences > General, and then her iCloud Photos Library in "Photos > Preferences > iCloud". Now there Photos from her own iCloud Photos Library should download to the empty Library, if she is signed into iCloud with her own ApppleID.