We can't help you - it needs access to your code running on your client's system, and we have access to neither.
So start by logging: add lines to print to a log file what if going on:
Dim OpenExcel As New OpenFileDialog
OpenExcel.Multiselect = False
OpenExcel.Title = "File Search"
OpenExcel.Filter = "Excel Files|*.xls;*.xlsx;*.xlsm"
OpenExcel.InitialDirectory = Path
Dim Result As DialogResult
Result = OpenExcel.ShowDialog()
If Result = Windows.Forms.DialogResult.OK Then
End If
Then when the problem has occured you get the client to send the file you you along with screenshots of the Open Dialog so you can look at them and maybe see what is happening.
But at the moment, you are just whistling in the dark - you need information to fix this!