Are you her coworker or the manager she reports to?
If you are a coworker, ask to have a one-on-one conversation with her. Once you are in private, express regret that you have to tell her something difficult. Tell her, in as nonjudgmental language as possible. “I frequently notice that you or your clothes have a smell, like cats or like they haven’t been washed.” Acknowledge that this is sometimes hard to notice on oneself. If you think it’s appropriate (her washer is broken or something, she’s living out of her car) offer to help. This conversation will probably take care of it in many cases, unless the underlying problem is pretty serious. If this doesn’t work, ask her manager to speak to her (see manager instructions). The advantage of taking this approach if you are the coworker is that you can avoid being someone who takes a problem to management. Also, your coworker will probably thank you (although she may not say it aloud) for not taking a problem about her to management. However, if her condition doesn’t improve, you should take the concern to her manager or your manager.
If you are her manager, do essentially what was in the coworker instructions, except with the addition of telling her that fixing the problem is a requirement of her job.
In either case, try to avoid gossipping among peers about the topic. That rarely helps the problem and often causes additional problems. Peer-to-person and manager-to-person conversations solve this problem.
You would not believe how often this discussion comes up on the HR message boards. This advice is based on years, perhaps decades, of discussion of the topic, with a pretty solid consensus among experienced HR folk.