I understand what your partner is going through. I went through the same thing EXACTLY, I was very close to my dog Barry and always had a feeling that something would happen to him to cut his life short. My father used to get so upset at me for being 'negative' in the end I was right both Barry and his sister Roxy died in a house fire last year, they hadn't reached 5 years old. Something inside me always knew that something would happen, I have had this once before in my life. When I was very small after my parents divorced and after my father's first girlfriend and him separated I had a feeling that I had a sister I didn't know... and I did. Some people are sensitive to these things and when you love someone or something so much and have these feelings/fears its hard to listen to others get mad at you for seeming 'negative' I am by no means a negative person, I think I am a sensitive person. The important thing to remember that despite having these feelings is that he needs to know that you have her today, so enjoy today like it is the last day with her.
I regretted and still do going out as often as I did without Barry and Roxy with me, not spending as much time with them as I could have (I was only just 21 when they died and was only 16 1/2 when we got them). We loved them dearly and it saddens me everyday that they are gone, but you have the opportunity to learn and teach your partner this. Enjoy what you have today because you may not have it tomorrow. Walk her everyday, walk her TWICE, play lots, cuddle her and talk to her, enjoy her for her and don't try to change it. I live by this now with my 2, we take naps together, go shopping together (except groceries! Hubby stays home with them) we walk atleast twice daily morning and night. I gave them a dog door so they have freedom, they are both rescues and I think about Barry and Roxy every day and thank them for sending me Tyson and Sable. I never take a day with them for granted and hug them and tell them that I love them and always will many times throughout the day.
Trust me, looking back and regretting is the worst, take my advice I lost so much to gain that knowledge. Reassure your partner that it is OK to worry SOMETIMES, but its not OK to dwell and fret. Love your girl and remember my words.
Colly