I've noticed a lot of talk about ADHDers being "self absorbed" which to be honest, really surprised me - having connected with lots of other ADHDers over the last year or so, the general vibe is that they make up some of the most empathetic, supportive, self-sacrificing groups of people I've ever known.
However.. I imagine a lot of this is because of our mutual experience and understanding of each others issues. But something that commonly happens in the peer to peer support group I'm in is, when one of us is struggling is, we step up and support that person. Not necessarily everyone in the group, often some of us have a lot of stuff already on, in that moment those people effectively ignore the whole conversation, but no one else minds because we understand that this is the way we are - we have good days and we have bad days. We don't lack compassion, nor do we lack empathy IMO, what we fundamentally seem to lack is an ability to be proactive - we're almost always reacting to something - whether it's our own feelings (hence appearing self centred), someone else's feelings, a notification on our phone, or the internal chaos in our brain.
I think where we really struggle is not so much, caring about other people - a lot of the time we care immensely (please bear in mind I'm speaking for myself and also generally, I'm not speaking on behalf of your specific ADHD SO at all), it's handling being asked to be something different than we are- to be proactive rather than reactive. Being told that we don't care because we struggle with proactivity. Sometimes this isn't even explicitly said, but it's been said so many times before that any kind of criticism of our now-centredness puts us firmly on the defensive.
I have a few ADHDers in my life that I have very good mutually supportive friendships with, so I really struggle with the idea that we're so self absorbed that we're unable to care about anyone else - and it's even possible to "hack our brains" into a crude form of proactivity by just, setting up things for ourselves to react to (calendar reminders always work for me, if it's in Google calendar it's happening whether it's a one off thing or a weekly reminder). I do understand why ADHDers might give off this impression though, and I also don't doubt that there are some ADHDers who are in fact very self centred - like I say this isn't intended to write off anyone's bad experience with an ADHDer, just a common subject I thought worth bringing up as it feels a little like we getting written off as incapable of empathy at times. I think in some ways it's very hard for ADHDers and non-adhders to have empathy for each other in either direction