Nothing much to expect. For the vast majority of people, it's a 10-15 minutes casual conversation about the applicant's life, and a check of IDs, either a very quick one, or a bit more in depth passport check in some cases where IRCC also needs a copy of all (translated!) stamps.
I believe IRCC asks for translations to be ready for the interview, so have those for the passport stamps of the passports your wife had in the past 5 years (including older stamps if within those passports), but that's pretty much it.
As for her residence, she has every right to be outside Canada during the processing, as long as she maintains PR residency requirements, so don't sweat it. She will simply explain her situation, and tell IRCC what her plans are for a return to Canada, so they potentially take this in account for oath ceremony, but that's pretty much it.
To be honest, unless there's something problematic that we can't predict, like your wife appearing to be unable to communicate in English despite the language proof she sent, there's a high chance the interview gets summarized as : "that's it? Spent thousands of dollars on a trip for this??"
