"After her death, we opened her testament."

Translation: Après sa mort, nous avons ouvert son testament.

[BillConlon]

why is "her" sa in one part of the sentence and son in the other?

[georgeoftruth]

Because "la mort" is a feminine noun and "le testament" is masculine. All possessives such as "sa" and "son" depend on the object, not the owner.

• sa mort = his death/her death
• son testament = his testament/her testament