My friend has really disgusting BO...should I tell her or not?

One of my best friends has absolutely terrible body odor. It's this putrid smell, and it's something that's lingered around for YEARS. I've never had the balls to tell her. She moved a few hours away in the last few years, so fortunately, I don't have to deal with her BO that often.

But every time she visits or when I visit her (like this past weekend), I smell that same, DISGUSTING, rank smell that's similar to when you don't shower for several days. I KNOW other people smell it too because they've mentioned it, and when she linked arms with another friend this weekend, the girl quickly unlinked arms with her within five seconds and the reason was all too obvious.

The thing is, her hygiene is good! She showers regularly and uses hair products and perfume...but the smell is still there no matter what. I don't even know how her boyfriend deals with it. I let her use my deodorant and had to throw it away right after. I really think it's a deeper, underlying internal issue, and it actually makes me concerned for her health. She's overweight and has a terrible diet - I rarely, if ever, see her eat fruits or vegetables. This smell is making me not want to be around her. I really think I should tell her, but I don't want to hurt her feelings (even though I know that's going to be inevitable). How do I break it to her kindly? Or is that a bad idea?