Hello @suerc Nice to e-meet you here. I am Scott and my mother-in-law suffered from dementia as well as my wife having many dementia-like symptoms while fighting brain cancer.
I would echo @colleenyoung comments. I know in the case of my MIL my father-in-law was in a huge amount of denial. My MIL would try and leave the their shared office with the UPS driver thinking he was her husband, drive the wrong way on streets, and not remember how to fully dress, but my FIL still remained in denial. My wife finally went back home until he ultimately agreed to have some help for his wife. It took her three weeks before my FIL came around to admitting his wife's needs.
I also agree the topic of the care of your dog may be an easier introducation to the issue than anything else since you can bring it up as something that is effecting you rather than just what you have observed or your concern over her absenses. No matter what, these types of discussions are never easy, but can greatly help at times.