Send a email to UK customs and explain and ask what forms are needed. They are quite helpful believe it or not.
Being sort of in the game the easiest people to work with are the people that ask for help before hand and have the right paperwork. It’s the people that don’t do any homework first and get all hassles of filling out forms past tense.
Get the forms, fill them out and get your friend to put his form in with the parcel and it should be OK.
As said most customs officers Love correctly filled out forms as it means less work for them
😉