"her assistance" is certainly correct and usable in written English.
You could use it in a sentence like, "I am grateful for the her assistance in helping me finish this project on time."
We thank Karine Mathurin, research assistant, for her assistance in writing manuscript.
So Davis stepped up her assistance.
Prayers seeking her assistance were followed by healing.
We acknowledge Ms. Y. Hou for her assistance of experiments.
We appreciate Dr. Li Zheng for her assistance and discussion concerning the APEX2 technology.
We are also very grateful to Dr Sylvia Hoblyn for her assistance during the experimental work.
The authors wish to acknowledge Emmanuelle Adrien for her assistance in coding the data.
We thank Celine Feger, MD (EMIBiotech), for her assistance in preparing the manuscript.
The authors thank Dr. Kaori Sakaguchi for her assistance in data collection.
We would like to thank Tina Mistry for her assistance in performing MIC testing.
We would like to acknowledge Neha for her assistance in editing and proofreading the paper.
