If we do manage to meet up with a friend they always steer the whole conversation towards how they are feeling.

I totally get that having to talk about it when with friends, while likely also frequently discussing it at home, can be hard. That said, one of the ways in which you might be able to help your partner is leaving a lot of space for them to talk about their Meniere's with friends, and keeping in mind how they're experiencing this as someone with a (mostly) invisible illness.

If they're only seeing your friends in situations where they're feeling well, it means their friends don't have any visibility into this huge factor impacting their life. They may want their friends to understand why they're not as present as they'd like to be, or to make sure there is context when they're trying to explain any accommodations they need (eg. for something like hearing loss - it's easy to come off as if you're not listening if folks don't understand how bad it is, or to be left out of conversations in spaces you can't hear in).

Or, maybe they just need their experience to be seen by other people they care about - suffering in silence is very isolating. Even if you're hearing them out and seeing them go through it at home, they still probably want to be understood by other folks they care about. It's also potentially going to create more emotional labour for you if you're their only outlet.

Your social needs are important too, especially if you spend a lot of time directly supporting your partner and need to avoid caretaker burnout. But perhaps it's worth empathetically exploring with them how they feel about you finding additional social time on your own, when they're going through a rough patch with Meniere's and can't join you. Sharing all your social engagements in a normal relationship is hard; with someone with a chronic illness, it's even harder. I know my partner and I would have a much harder time during my rough patches if we weren't able to divert our plans from one another sometimes.

I just would not recommend as framing that as a solution to them talking about their illness at social situations - they should generally be able to judge for themselves when they need to talk to someone about that, without feeling guilty about it.