Unless I misunderstand your question, you can just open a file read only. Here is a simply example, without any checks.
To get the file path from the user use this function:
Private Function get_user_specified_filepath() As String
'or use the other code example here.
Dim fd As Office.FileDialog
Set fd = Application.FileDialog(msoFileDialogFilePicker)
fd.AllowMultiSelect = False
fd.Title = "Please select the file."
get_user_specified_filepath = fd.SelectedItems(1)
End Function
Then just open the file read only and assign it to a variable:
dim wb as workbook
set wb = Workbooks.Open(get_user_specified_filepath(), ReadOnly:=True)