She is an older woman that has a problem that has already affected some of us personally. She leaves some of the office Chairs "wet" and pretends like nothing is wrong. Someone has accidentally sat in the "wetness" several times! Not only is that unsanitary but the smell, OMG! Please Help!
This is becoming a health issue, and someone needs to speak to her about it. It could be HR, or her supervisor, or even you; but it must be done in a dignified and respectful manner.
Beyond that, she must really take the lead on being responsible enough to care for herself; and to stop being selfish in ignoring her coworkers legitimate concerns. She needs to seek medical care, wear protective pads, and use absorbent mats on the surfaces she sits on. This is not optional, for all of your health.
Cheers!