Just wondering when writing in Spanish whether one would write:
sus vidas — "their lives"
su vida — "their life"
e.g. "it changed their lives" or "it changed their life", when refering to multiple people.
If "su/s" refers to "they", the singular will indicate that they have something in common (their life together), while the plural will point to their having individual or separate lives.
Su vida cambió a partir de que se casaron. (Their life -- their joint life, their life as a couple -- changed after they got married.)
Sus vidas cambiaron a partir de que se casaron. (Their lives changed after they got married.)
The way we express "This diet changed their lives" in Spanish is
Esta dieta les cambió la vida.
Esta dieta les ha cambiado la vida.
You could make it a little more dramatic by adding "para siempre" if you like.
If you want to see some examples of natural usage online you could look for
la biblia me cambió la vida -- that's a very common one.
Sure, one does sometimes see formulations such as "cambió sus vidas," but that is not idiomatic. It is the result of the influence of English, and it's not the natural way to express this in Spanish.
Both are correct (see for example here). I would prefer the plural form (cambió sus vidas) to avoid ambiguity, especially in this case (third person) where there is no other plural indicator in the sentence.