Two Paws Down to New Kitten

I got two kittens to keep my adult cat company who had lost her sister. The kittens were too frisky and disruptive for my adult cat and me.

The cat lover did not mention exactly HOW their cat was acting toward the stray - the word 'jealous' is subject to interpretation.
The doctor mentioned time and patience, assuming that the cat lovers wouldnt be willing to try! If the original cat is demonstrating overt or covert active aggression behaviors, then it might indeed be a happier household if the 'new guy' could be fostered and re-homed elsewhere.

However, gradual, brief introductions (behind closed doors, screens, baby gates, or carriers) over two weeks to a months' time is a good initial measure of how tolerant a cat might be of a 'new guy'. Some demonstrations are normal and brief - most multiple cat homes have seen a good amount of growling, puffing up, and attitude in the beginning. Let Original Cat and New Guy take it SLOW! Smell-sharing, with bedding, toys, and towels is helpful and non-intrusive. Using Feliway spray and diffusers in Original Cat's main areas can help friendly feelings, as well as Bach's Flower Essences in drinking water. After a few weeks of familiarity, try Original Cat and New Guy together in the same room (- do keep a squirt bottle handy, but only use it to distract an active aggressive behavior like biting, otherwise the cat will associate the squirt with the other cat , or you). Continue the gradual togetherness slowly . . . the benefits of companionship can be worth it for both Kitties in the long run - they might eventually thank you for your time, love, and patience.