When I was in my second year of music college, I had a platonic friendship with a young man who was a year younger than me. We were in a practice room one evening and he was humming without opening his mouth. He was complaining that he couldn't get a girlfriend and wondered why not. It was at this point that I took the plunge and, as gently as possible, mentioned his distinctive aroma. He was living in digs and thought he was only allowed one bath per week. I suggested that he could ask his landlady if he could have two baths a week. I also told him that he needed to change his underwear every day. He took it very well and, a few days later, told me that he had bought some new underwear. He never had the problem again.
It is an extremely difficult issue to air and many people will take offence, but it is a conversation that needs to be had occasionally and most people will take heed, even though they don't like being told about it. It is the old cliché about being cruel to be kind.