You should talk to your HR team first. They are ideally placed to handle it and are trained for this kind of confidential encounter. They should be skilled at doing it with minimum offence and maximum sensitivity. HR training encourages pragmatism and coolness in a crisis. That approach should help this junior colleague find a solution to the problem while being assured that, once it’s corrected, it is something that won’t affect their future within the company.

If the HR option is not one that’s available to you, then the next best person to deal with such a problem is the line manager. I worked in an office once where exactly this scenario occurred. I was pretty junior myself at the time so was not privy to the innermost details of the sensitive conversation but the line manager definitely had a private word with my malodorous colleague and pretty soon the rest of us were aware that our office had suddenly become much sweeter smelling. The problem never recurred and never got mentioned again.

Honest Boss: how to tell someone in your office about their body odour without making things awkward