Many years ago, when l was beginning the second year of my degree course, I made friends (platonic) with a young man who had just started. I noticed as the term went on that he was beginning to get a distinct odour - a mixture of BO and unwashed clothes. We were in a practice room, which was small, and I noticed the smell more than ever. He complained to me that he couldn’t seem to attract a girlfriend. This was just the opportunity I needed so, taking courage in both hands, I told him what th problem was, adding that it would be a good idea to have a bath more than once a week and that his underwear needed to be changed every day. He was very pleased that I told him and, a few days later, told me that he had bought some new clothes. He never had the problem again.
As for your friend, it is, indeed, much more difficult as she is older. I think that l would support the answer suggesting that you had noticed a smell of wee that day and were concerned for her health - could it be a UTI, kidney problem or diabetes? If you make it sound as if you were concerned about her health rather than the smell, she might be more receptive. You could also add that when you had a UTI that you found X brand of pads most helpful.
Horrible smells are so difficult, aren’t they? For me, it is the smell of cigarettes that is worst and l find that if I am in a queue behind a smoker (always noticeable by the smell on their clothes) I try not to breathe in too much and take a step back.