If you add the implied verb, you would say: "she fell behind her friends since they are more experienced than she was.
Deleting this implied verb, the sentence would end properly "they are more experienced than she."
I'm not saying this to be Freep grammar cop, I'm saying it to point out the effect of ESG and DIE on once great newspapers. Don't believe a word they write.
But are you sure the rider was identifying as female at the moment? I'm not saying this to be Wokeism grammar cop, but using the wrong pronoun may cost you your job faster than leaving a person on a lift overnight.
/Sar