First, have an open, honest conversation with them one on one. Ask them WHY they are working so hard. There could be work cultural influences, they could have too much work on their plate, they could have a “I’m the only one who can do this” mindset. Who knows.
So, just ask them to spend some time processing that question and then come together and strategize how to get back to a more reasonable situation.
I’d also encourage them to get some help. I’m not sure therapy is necessarily needed, but a good life coach can be a HUGE help to uncover better strategies for living and working in balance.