The short answer is no, don’t lend money to an estranged adult child. If you can’t afford to make it a gift (or if you choose not to), it may be better to avoid any financial involvement.
If you can help it, try not to become your child’s creditor.
As a creditor you’re on the hook to take action if your child defaults. This doesn’t bode well for a good outcome when there’s already tension in your relationship.
What should you do in the event that you already loaned your adult child money? Now she’s estranged and not paying you back.