Upvote:1
Your friend breached the conditions of his visa and departed voluntarily after the initiation of removal. The documentation he received would have indicated the applicable re-entry ban, e.g., 12 months if the individual leaves the UK voluntarily and at their own expense.
He certainly can apply for leave to enter the UK, and must if he is a visa national, should if he is a non-visa national. While a non-visa national does not normally need advance permission to enter the United Kingdom, it would be advisable to do so with such an immigration history.
Understand that his breach and removal are part of his record, permanently, and will be considered when he asks to be allowed to return to the UK.