My DD was in hospital for 3 months from birth, I had had a CS. Obviously I did not hold DD in a sling however when DD was 10 days old she was transferred into the city, I walked half an hour to the station with a backpack containing bottles of expressed milk, bottle of water etc... then on a train, tram and walk a couple of minutes into hospital. I did not have any discomfort and had no problems, and some days I ever took the stairs to level 3 as the lifts were sooo slow.

Ok so what I went through proberly isn't recommended but what I am trying to say is I think that she will be fine to wear the sling for small amounts at a time. I am no doctor though just my opinion, but I did have an unusually quick and comfortable recovery.
I think your wife will know if it is to much or not, just wait a couple of weeks after the birth to try.

Good Luck.