Q.mono grass
I have mondo grass that was planted under a tree and it did really well. The tree has been cut down and the grass is exposed to full sun. It is beginning to thin out. What can I do to save it other than replant in another area
I would suggest dividing your plant and moving the new divisions into a partial to full shade location.
Here are some articles to help you.
https://www.gardeningknowhow.com/ornamental/foliage/mondo-grass/dwarf-mondo-grass-propagation.htm
https://www.gardeningknowhow.com/ornamental/groundcover/monkey-grass/monkey-grass-a-spectacular-groundcover-for-the-lawn.htm