Here's a legal perspective.
If the smell is so bad that it is affecting your health/ability to do your work, and if colleagues are affected too, that is not something you should be required to put up with for fear of being considered discriminatory.
As soon as your employer is aware of the extent of the problem, and the fact that it is not a temporary issue, they are under a duty to act.
They should have a discussion with your colleague to explain the issue, in case she is not aware of it. They should discuss with her strategies which might help to reduce the problem, to see if this is an unavoidable consequence of her condition. If the employer was my client, I would advise paying for a private consultation with a continency expert/continency nurse. The employer should then consider strategies to address the problem. For example, if frequency of pad changing is an issue, can anything be done to make this easier for your colleague - eg a designated loo, moving her work station nearer to the loos etc? This should all be approached in a sensitive way, but the employer should make your colleague aware that this is not just about her, as the problem is creating an untenable working environment for others, and so needs to be addressed for that reason as well.
If your colleague refuses to engage with this process, or if, despite all of the efforts above, the smell is still unbearable, then, ultimately, your colleague's position may become untenable. After consultation, and probably an independent medical report (if she will consent to it), her employment could be at risk.
I know some will say this is harsh, but I am not suggesting that this would be appropriate if the smell was only slight and/or occasional; looking past that would, IMO, be a reasonable adjustment to accommodate the effects of the employee's disability. There comes a point where an employer is not required to accommodate insurmountable problems which are seriously affecting others, whatever their cause.
For that reason, if the problem really is making your work life so unpleasant that you are considering leaving, you need to tell the employer that, not simply pretend that you are raising the issue only out of concern for your colleague. You would need to point out to your employer that you require them not to "implicate" you as the person who mentioned this.