Card renewal and new applications take quite a bit of time for some, and not a ton of time for others. If you look in the nexus renewals thread you'll find that many people are getting it renewed without an interview. This may be the case for you as well. In fact, there's a post in that thread linking a news article stating nexus members have 5 years from conditional approval to schedule an interview

There's a five year grace period to the expiry date on the card once a renewal application is submitted for a not-expired membership. I'm not sure whether it has to be submitted x days before expiration, but you could theoretically submit the application close to her expiration time and continue to benefit from PreCheck etc.