Contrary to his authority for preventing.... I guess my view is that the Prime Minister, and any minister, has no authority to prevent someone from appearing in front of a committee.
Their ministerial function may present a limitation on what you can ask that political aide when they're in front of you, but everyone has a duty, apart from members of Parliament, senators, and the Governor General, to show up when summoned before a committee. What questions they can be asked is another matter.