International Relations at St Andrew’s and applying for PPE elsewhere

We visited St A with my son today, he is looking to apply for a 2024 place to read International Relations whilst making applications elsewhere for PPE.
The Admissions Manager told us that it will be a straightforward rejection if PPE is mentioned in the PS. This leaves us with a dilemma,. St Andrew’s is the dream Uni for him, on predicted grades he has every chance, but the acceptance rate is low and he can’t throw away his chances with his other choices.
How can he write a Personal Statement without alluding to the components of PPE or Int Rel?

Hi, I am not sure if your son has already submitted application, but here is one option: he can apply to the joint honour programme of IR at st. andrews through CommonAPP, which won't affect his application in the UCAS. Then, he can apply to PPE at other British Uni while applying to St. Andrews IR.

Thank you, I’m not familiar with Common App