How to help my partner with grief from years ago

So my partner's older brother died in a traumatic way a few years ago and it has been very badly affecting them ever since. Every time death is brought up or something about him is brought up they go into heavy dissociation and become severely depressed and I didn't understand how bad it gets until it got to the anniversary of his death.

They go to therapy for other mental issues but I don't believe they've talked to a therapist about the grief. I know they want to get better and be able to deal with it easier, and I'd like to help them in any way I can, although I'm not entirely sure how I'd be able to. I definitely am going to try to remind them to mention it to their therapist. They heavily avoid the subject and they only really feel safe talking to me about stuff so I'm hoping I can find some small things to do with them to help them actually grieve and deal with their brother's death instead of completely shutting down mentally and physically and dissociating until they don't have to think about it.

I know there's not much I can do unless they actively want to get better, they've said to me they want to but they don't know how. I was thinking maybe I could get them to talk to me about him, what kind of guy he was, his interests and just some facts about him just to get started, I only know he was older (and how he died which I don't know is appropriate to state here), but I would like to learn about him to both preserve his memory and help my partner through all of this.

I have their permission to post this, I wanted to make sure that it was okay with them before posting. I am VERY sorry if this is not where I should post this, I don't know where to go with this and if it shouldn't be here I will delete it as soon as I'm made aware or mods can delete it (and hopefully tell me why and where I should take this if they know). I just would like any advice on how to help my partner, I love them a lot and I want to be able to see them get better and be able to actually live their life.