My wife's calendar is shared to my calendar and then synced to my phone through my account and I currently get reminder cards for her events in Google Now. I would like to only get cards for certain calendars.
Apparently, Google Now uses the "My Calendars" section of Google Calendars.
When my wife's calendar was shared with me, I was granted "Make changes AND manage sharing" rights. This puts the calendar into "My Calendars" for both users' calendars. When I changed it to "Make Changes to events" on my wife's sharing tab, the calendar moves from "My Calendars" to "Other Calendars" for my account and I no longer get notifications in Google Now for that calendar.
Instead of synchronizing her calendar to your phone, try setting her calendar "shared" and then subscribing to it from your own Google Calendar. When you subscribe, I believe you can set it to not pull in the reminders.