So, this is tricky. One possibility is that they may not have stable bond currently because Sage isn’t spayed, or even if they do, her hormone fluctuations post-spay could trigger fighting. But I’ve also seen cases like this where the buns go right back together no problem.
How do they behave together now? Do you ever see excessive mounting, chasing, fur pulling, or nipping?
Also, are you able to supervise them closely for several weeks after her spay? There can be hormone fluctuations for 4-6 weeks.
If you aren’t able to supervise closely, the safest thing would be to house them side-by-side after her spay for a month or so. You can also do side swaps and even let them interact in neutral territory while supervised (once she’s fully healed). They won’t forget each other that way and will be able to keep each other company, but there won’t be the risk of a fight breaking out. A bad fight could make it harder to re-bond them.
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.