I've been the person being complained about. I've said before, but I have strong body odour. Bad enough that I have to change my socks and shirts because I can be smelt. Sometimes, I have to change 3 times a day. It gets better. Because I have escalating thyroid disease, I have no control over internal body temperature, so I often wear a fleecy jacket. Naturally I sweat in it. I've purchased new jackets twice in the last year.
What's this got to do with the topic? Because everywhere I've worked, no matter how clean I am or how many showers I have, people have complained about the smell, usually late in the day. And there is no cure, it just has to be managed and controlled. Most bosses have been good about it and understanding that I know about it and do what I can, but I have had one or two that think I'm doing it deliberately.
So I'd suggest speaking to the person involved first, quietly and to one side. There might be nothing that they can do, and if it's after 3pm I wouldn't even bother commenting. But the last thing I'd do is speak to their boss, who may see this as an opportunity to cause problems for the employee.