I know a bit about this having gone to one recently (my cousins)
Firstly, depends hugely on how religious they are etc.
But if they are reasonably, then you will have to cover your head, would recommend simply a scarf for that, unless you actually have a proper headscarf.
If I were you a simple long skirt and blouse would be your best option, its perfectly acceptable for non-muslims to come wearing something different and simply dress conservatively. But if its in a mosque, wear a headscarf.