How to support a grieving person?

Hi everyone, I am writing this in hope to get some support. I have, quite literally, never lost a loved one. Yet.

I've been living with my partner for some time now, and we both take care of their cat. She's the sweetest thing. They took her from the streets and they've done everything the vets ask. The poor cat was in a terrible state when they found it, but now over a year later she's doing much better.

The problem is, the vets were always super upfront with this. She probably doesn't have much time left. She had cancer and we had to have her ears amputated, but they said it was very likely to spread to other areas.

Recently we noticed her nose had the same stuff as her ears. And the only option is Chemo, which we agreed to not do it.

I know her death is inminent, and I am super scared. I'm gonna be heartbroken, but most importantly, my partner is gonna be as well. They have such a deep connections with animals, much stronger than anything I've ever felt or seen. I know our cat's death won't be "just a mascot passing away" for them.

So my question is, how do I support my partner when the time comes? How do I make sure that they don't spiral into anything dangerous while making sure they keep up with uni? I'm very new to all of this. I'm not sure how impactful grieving is, I'm so scared :(