It depends how the exercise was presented:
Dictation in French: "son" is most probably "her" husband
Marginally, it could also be "his husband" (Isabelle doute du mari de Pierre). But in France, please wait a bit since mariage between people of the same sex has been legal only for a couple of months ago!
- extra meaning: even with "her husband", it could be another woman's husband (Isabelle doute du mari de Jeanne). Context would tell.
Written French for translation into English: see above
English text for translation into French: only "her" (+ extra meaning)