I am making a madeira cake for my birthday on 18th August. I shall add glycerine to help keep it, but the person who is going to decorate it for me is going on holiday, so he needs the cake by 8th August. I have tried making and keeping a couple of madeira cakes in the past few weeks, but they have both gone mouldy within a week. Would it be more advisable to make a fruit cake instead?

I think you can do either this madeira recipe that Lindy says lasts 2 weeks from baking, or as you say opt for the fruit cake, as that can last ages.
The ingredients in the Lindy's madeira may be different to the recipe you have been using and may help it live for 2 weeks.
http://www.lindyscakes.co.uk/2009/12/17/baking-the-perfect-madeira-cake/

Enjoy!

I would think that you could freeze the cake and then the person decorating it could keep it in their freezer to take out maybe the day before decorating it. HTH