If the duvet is a solid red colour you can work with very nearly any colour you like. I would recommend a medium to pastel colour, but let your personal taste help you make your choice.
If the duvet has a pattern in it, you can choose any of the colours in the pattern as a starting point and tone it down or up as you choose.
I do recommend you use a bit of black in your room, as it will help to ground the room and black and red always look great together. Try small accessory pieces, or if you are feeling adventurous (and have an okay from the owner of the house) try a black floor or area rug.