Well, it is not well-formed :) See that Visible attribute there. Your attribute is enclosed with " markers so you can't use " character inside the attribute. This is a restriction of XML: http://www.w3schools.com/xml/xml_attributes.asp
So what you can do is to write it like this: Visible='<%# ShowOrHideRow(Eval("Manager").ToString) %>'
Or like I'd do (if Manager is of string type): Visible='<%# (string)Eval("Manager") == "Y" %>'