Hi Dusty, I bought my hammock second-hand and it is so easy to keep clean. The whole fabric section and foam mattress can be machine washed. If she is worried about the mattress she can buy a new one directly from Amby. The owner Ambrose is really helpful. I sell second hand baby goods and have bought new mattresses and spare parts from him before. TBH I didn't buy a new mattress for mine because it can just be washed in the machine.
Tell your sister to look on eBay for one or even contact a baby hire place because sometimes they sell their older ones (that's how I got mine
)
HTH 