You sound like such a wonderful Mum & Gran ... I really feel for you. I tell you my own story, which hopefully will help you in some way.
I know you are concerned, but you need to tread very carefully. My own situation with my Mum was quite similar a few years ago. And because I am so close to my family, it was a very hard time for me.
When my kids where very young, my marriage went through a rocky patch. I wasn't happy. I know my DH wasn't happy. We were both exhausted trying to raise our 3 children and work at the same time. We didn't have anytime for each other. Our once happy marriage began to suffer. Our sex life came to a halt, and as a result we were no longer emotionally close.
My Mum was concerned and I know she blamed my DH (!). She began to take things he said the wrong way. Misunderstandings happened. Family occasions like Christmas became a strain. I know my Mum talked to my Sister about it. I began to see my Sister's contempt for my DH in her eyes. Even though I never spoke to my DH about it, I could sense he knew my Mum & Siser disliked him. I didn't want to confirm his suspicions, because it would have made an already strained situation much worse.
I was soooooooo unhappy. I got very drepressed about it because there was no-one I could confide in about it. I was piggy-in-the-middle. I never spoke to my Mum at the time but this is what I wanted to say to her:
"Please back off! I love you and I know you mean well, but please leave it. Don't try to fix it. Things will get better. Its hard for us right now but it won't always be like this".
And I was right. It did get better. Now the kids are getting older, things have improved immensely. Family life with our kids is now joyful. DH & I have time for each other again. Our love for each other never died, but it definately lay dormant for a few years.
The relationship between my DH and family is a lot better too. Not perfect yet, but we are getting there.
What I want you to know is this. I knew why my marriage was going through a rocky patch, but I knew we would come through it. As hard as it is, try to trust your daughter's judgement. If she feels her life with him will never improve, she will leave him, but in her own time. Believe me, no-one knows or feels it more than your daughter. There is no need to make hints or point it out to her, she already knows!!!
I also know from our dark years that people become negative when they are unhappy. My DH was negative and so was I (but my family only chose to see my DH's negativity). Now that our lives are happy again, we are living with a positive frame of mind.
Try not to be too judgemental of him. And don't put any pressure on your daughter. Be the most wonderful & supportive Mum you can be. This is what your daughter needs right now more than anything. Make this your focus. Don't let her know you are worried sick, that will only increase her already burdened plate. Trust her to do the right thing. Good luck.