Upvote:2
Can he go back to UK as a visitor?
Yes he can, nothing in the law/guidance prevents him from.
Are there any specific requirements/risks?
Yes there are risks and heightened suspicion from immigration and UK Border Force however not much different from anyone who previously had a dust-up with UK immigration particularly since he did not overstay.
He can apply for a visitors visa (typically the preferred panacea recommended here with less stress), or he can choose the visa free route like I did and be ready and well prepared with ample documentation to overcome the inevitable grilling ahead.