I did some research and came to several potential conclusions, depending on exactly what the author of your book intended. I'll share what I found and let you decide what fits best for what you were reading.
My First Conclusion
This is the correct sentence:
He was performing the same steps as she
From Lexico Definition of her
Is it incorrect to say I am older than her (rather than I am older
I think your example is the same as "I am older than she."
Both sentences compare and contrast:
- I am older than she.
- He was performing the same steps as she.
Then again, as I keep digging, I discover that it's not so clear-cut.
Lexico Definition: Personal Pronoun
It seems that it is normal to say:
I didn't do as well as her.
I know I talk like that and so do the people around me. We also say, "I didn't do as well as she did." English is very flexible. But if that sentence for Lexico: Personal Pronoun is correct, then so is the sentence:
He was performing the same steps as her.
But it should not be "hers" because that is possessive.
From What is the difference between “her” and “hers”
My book is better than her (book).
My book is better than hers.
It is cumbersome to say "My book is better than her book," so we say "My book is better than hers."
To compare the sentence about performing steps, consider which of the following two meaning is intended:
- He was performing the same steps as her steps.
- He was performing the same steps as her/as she was.
If the first, then it is correct to say: He was performing the same steps as hers.
If the second, then it is correct to say: He was performing the same steps as her. You read the book; you know which best fits.
It's very late and I'm not sure if this is clear. Let me know if you have questions.