Q. In a sentence like “the authors thank Natalie and Isabel for her editorial assistance,” is it grammatically correct to use the pronoun her and not their?

A. If the authors intend to thank both Natalie and Isabel for assistance, then their is the right choice. However, if the sentence means “The authors thank Natalie [for something other than assistance, but we aren’t saying what] and [we also thank] Isabel for her assistance,” then even if it is technically grammatical (debatable), it is nonetheless confusing. (Correct grammar does not mean everything’s OK. “Striped sentences wish green habits” is grammatical.) In short, your sentence is a disaster and must be rewritten for clarity.