They're only typical offers, and you can get offered on 4 subjects if you take four. I was told to be wary of that when I took 4 AHs this year by my maths teacher as someone the year above had been given an offer on 4 subjects for Imperial (not Cambridge but similar entrance requirements usually). Of course, nothing wrong with trying!
On the happier side, good it's this early, whilst it's not great to have to work on another subject after leaving it behind, it's early enough in the year that with some work she can 100% achieve it.
I wish her luck! And congratulate her for her achievement on getting in.