It's actually my MIL, so DH is dealing with the brunt of it, but I'm also heavily involved. The big difference is that he has a massive emotional loss, whereas I'm sad but not as devastated IYSWIM.
We're not quite at the stage of not being able to have meaningful phone calls, but it's close. We don't hope to achieve much from phone conversations with MIL other than chit chat and perhaps a bit of talking about what she's looking forward to, but we do still try to make the most of them.
We also do a lot of redirecting towards happier topics, and do our best to avoid arguing. There's a fair amount of... not so much lying... but avoiding giving answers that we know will upset her. I think that becomes more of a strategy you need to use as their memory gets worse.
A frustration is that she forgets things are going to happen. She forgot DH was coming to visit, and arranged something else that week. We often FaceTime her and have her actually open her diary so that we can get her to check if something is written there.
She does know to check her diary regularly, at this point, but now if we're planning to visit we know to contact a nearby relative and get them to check there is some clean bedlinen, she has some food in the house, she hasn't arranged to be somewhere else.
We have a lot of conversations with other family members to check up on how things are going, and follow up on things we're worried about.
We are at the stage where we get the same text messages and calls every day about the same subject, often something she's worrying about and has fixated on, over and over. It is frustrating, and requires a deep breath, then having a script for responding eg 'Yes mum, I contacted the bank about that, so it's OK for you to stop worrying about it."
Sometimes saying the same thing over and over can help embed it in memory, so it's useful to have a script in place for difficult or repetitive conversations. Also for making suggestions eg "We're looking at getting someone to help you around the house a bit. Remember how much that helped your mum and dad when they were in their 80s?"
I'd really recommend reading some books about dealing with dementia. And sending sympathies - it's really tough. I hope your trip goes OK. I'd honestly try not to set your expectations too high.
I remember visiting my granny just before she died. Her memory was fine but she was dying of cancer and a lot of it was very upsetting. I'm still really glad I did it - some of my memories from that trip are the most important things to me - but it was good that my mum warned me of the things that would be difficult.