I am afraid that the provided information so far is insufficient, that is why I would like to ask you to answer us some additional questions:
1) Does the issue reproduces only on one user's machine with Windows7 or you are able to reproduce it in other machines with Windows7?
2) If you are able to reproduce it on other machines using Widndows 7 could you please send us a small runnable sample project that reproduces the issue? Another option would be to provide us with code snippets with the definition of RadGridView, the code that is used to hide the columns and any other events used.
3) What version of the controls are you using?
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