For clarity, your son has the children 75% of the time or is this your estimate based on how much you are examining and scrutinizing what your former DIL does with her time? I find it hard to believe she's personally giving you the rundown directly on all her plans. Is your son complaining about having his own children? If so, is there a reason he isn't communicating to her? Your son has other things going on in his life and is simultaneously parenting. This is life for literally every parent on Earth, so I am not sure why you seem to be putting him in a pedestal here for that. Being a man doesn't make multi-tasking in life any more impressive than if he were a woman. "He will be on his own with the children." Is your son a teenager babysitting other children or is he a parent? He chose her to marry and have not one but two children with your former DIL. It's his life to sort. If you don't want to have the children, don't. Everyone here is making choices. It would probably help for you to not be so enmeshed in your son and DIL's lives to this extent. Perhaps taking a step back and finding interests of your own would help you to reign in your preoccupation with your DIL's life and your son's. Enmeshment with adult children never helps anyone involved. Your son has to find his footing, be a father, and do whatever it he needs to ensure his children are safe, secure, and taken care of. Outside of moral support, I am not sure where your role is in all of that.
How you raise your children has nothing to do with your DIL or her life. Please stop judging other mothers for making different choices. If there is harm being done to the children, there are authorities you can report your concerns to. For your own peace of mind, try to focus on your own life. You are doing yourself no favors fixating on your DIL.