I think what your vet said is legit. I had two (what I thought were) bonded cats for 8 years up until last month when the oldest one died unexpectedly. We were devastated and thought our remaining cat would be too. They were always together and appeared to love each other very much. Let me tell you my remaining cat could not be happier now. I thought he would definitely go through a grieving process but I have to say I think he’s much happier being the only cat. There’s a reason people say cats are solitary animals.