Help with fine curtains!

I stained my grandmother's curtains with a sauce that contained mayonnaise and other stuff, don't know exactly what... The deal is that the grease stain is quite big and I don't want her to notice because she really cares about those curtains. I managed to hide the stain somehow by repositioning the curtains but it won't last long. What should I use to clean them up? 

Waiting for your answer, thanks!