Don't do it!!!! Online bookings are not feasible for the beauty industry. Clients don't always understand how the duration of a service is calculated and they also will attempt to book in such a way that will leave small gaps between her bookings resulting in her loosing valuable time.
Also you need to verify each customer before they can access the booking system to prevent new customers not pitching up.
Then your cousin needs to also manage all her bookings online, even instore to prevent double bookings - something that can ruin her reputation.
Oh, BTW, my wife has a hair salon and 11yrs ago her salon was suppose to get an online booking system (she was going to do the beta testing for a national system). My wife saw all the faults with the system even before she was suppose to use it and never did. Also the developing company of the product also gave me some insights into the problems they faced with the design process. I'm a PHP developer (specialise in callcenters) and understand the technical challenges.
Lets just say there is a reason why nobody offers this is a SAAS. I ended up just developing a basic database that is customised to her needs.
Your better off just developing a basic website where clients can access her contact details, her trading hours and her price list.