I rent one chair. It is best to keep it as a separate business from the salon, for vat and tax reasons.
She buys her own stock, handles her own bookings and payments and sets her own hours. She also launders and provides her own towels and other equipment.
I take a set weekly rent, this covers all utility usage.
The vat and tax rules are complex on this subject, we found that setting up and running in this way kept everybody happy.