Your friend is right: No helmet can prevent concussions. There's no way to keep the brain from moving inside the skull. If you hit your head hard enough, your brain can bang into the hard bone and cause a concussion. Some "special" sports helmets and other new products claim that they can protect you from concussions.

Does headgear reduce concussions?

Conclusions: Padded headgear does not reduce the rate of head injury or concussion. The low compliance rates are a limitation. Although individuals may choose to wear padded headgear, the routine or mandatory use of protective headgear cannot be recommended.

Does headgear prevent concussions boxing?

Headgear doesn't protect boxers from concussions. ... Counterintuitively, referees had to stop matches for head injuries (aka likely concussions) more often when boxers were wearing headgear, according to an AIBA study.