Because, the parable is telling us, if we are not healthy and taking care of ourselves, we are not going to be able to help others. So you have to apply this to your situation:
You and David have to be able to be on solid footing financially for your own later years. This is a requirement of life today. No one else will do it for you, not his kids or anyone. This is the advisor's point in recommending not borrowing against the retirement funds.
David needs to be able to see this and accept the advisor's recommendations. But he may be too swayed by his feelings of guilt as I described above. Parents of divorce often feel a tremendous sense of obligation born of guilt. If so, then you must take the steps with the financial advisor to protect yourself. That might mean a nuptial agreement separating your savings and retirement funds from his obligations and protecting assets that are in your name.
I know this sounds cold and unloving to you. But remember what I wrote above: you MUST give voice to the legitimate needs of your lives. If David cannot share in taking care of his needs, you must protect yours. I say this with respect for David and experience working with adult parents of divorce who feel such obligation. It cannot dominate his life; and it must not dominate yours.
So, please have David read my answer. It's said with caring for your situation and recognition that his kids are good people and have legitimate needs to invest in their future. But he is one human being and they have to live within the limitations of their situation. If that means they have to become self sufficient before they had hoped, that is sometimes the situation young adults face. But you and he have a right and an obligation to your own lives. So please have him read my answer and discuss it, maybe even with the advisor.
Okay, I wish you the very best!
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