I do not know for certain but unless IRCC specifically suggests or requires a leave letter, then don't overthink it. From their perspective, the key is whether she has a job she is likely to return to. (I don't have any information about how they will evaluate her specific file)
Once issued a visa, it is her responsibility to obtain leave from her employer; ultimately IRCC does not much care if she can't get leave and doesn't end up coming to visit.
That said, if for some reason you think it strengthens the application to get a letter specifying that they will give her exactly, say, two months to visit her husband, then by all means, provide it.
The other side of the coin is that if they don't believe that her employment is sufficiently compelling for her to return at the end of her visit, then I doubt a leave letter will make much difference.