You can easily manipulate either horizontal or vertical Gridlines in your RadGridView/RadTreeListView as it is shown in our online demo-GridLines.
However if you want to change the Thickness of your GridLines,you can edit the template of the GridViewCell,there you can find the following part,named "PART_CellBorder" and increase the thickness on the right side in the way you need.
Please let me know if you need any further assistance.
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items