I'm not a doctor, but I'm almost certain your SO absolutely needs to discuss these things with someone to have any chance of relieving, or reversing the trend you mentioned.
From my past experiences, I must unfortunately say that you might be just about the last person they want to, or can discuss it with (as strange as that sounds), but without them confronting their habits, it is unlikely it is going to get easier. Even with crippling forms of OCD, it is easier to just "comply" than to challenge it, the latter being "guarded" by great anxiety that feels much more intimidating than even the most time-consuming and difficult routines, even if they fully realize some of the irrationality. (That, too, may be hard to understand from another person's view.)
From my past experiences, I must unfortunately say that you might be just about the last person they want to, or can discuss it with (as strange as that sounds), but without them confronting their habits, it is unlikely it is going to get easier. Even with crippling forms of OCD, it is easier to just "comply" than to challenge it, the latter being "guarded" by great anxiety that feels much more intimidating than even the most time-consuming and difficult routines, even if they fully realize some of the irrationality. (That, too, may be hard to understand from another person's view.)