We went through something similar last year. My lovely MIL, who had lung cancer, secondary to pancreatic cancer, entered her final illness. We saw her in the summer, when she came to visit us, and even then, the change in her was obvious - she was not the energetic lady her grandsons knew, and looked very different too - ill and much thinner.
When she went into the hospice, in early winter, ds2 and I flew down to visit her, meeting up with ds1, who is at university in the south of England. We offered ds3, who was 17.5 at the time, the opportunity to come with us, but he really didn't want to go - he said he didn't want to see his grandmother looking even sicker than she had when she'd visited in the summer - she had gone downhill significantly since that visit. He said he had found it very distressing to see how ill she'd been in the summer (though he had concealed that from her and from us at the time), and he knew he couldn't take seeing her much worse, and in the hospice. He said he wanted to remember her well and happy, and didn't want the memory of her in the hospice as his last memory of her.
Dh and I respected his decision - he had good, well-thought-out reasons for it, and so he stayed at home. He did talk to his grandmother on the phone on a number of occasions, so there was plenty of contact, and she knew how much he loved her. And over the following weeks, whilst she was in the hospice, and then when she was discharged home to die, we made sure he knew he could change his mind and we would make sure he got to see her, if he wanted to - but he didn't. I am absolutely sure it was the right decision for him - and I would not have dreamt of forcing him to go and see her.
He and his brothers helped my dh to carry my MIL into the church for her funeral, and I have never been more proud of any of them.