You have to refund the full amount she paid.
The debateable bit is whether you should refund the postage cost to return.
Some say you should send the return postage, some say - M&S don't give you your bus fare if you take something back so why should ebayers...
Would prob be easier to just refund for this amount rather than argue the case - unless of course the stain can be proven not to be there in the photo you uploaded - but even then it could be argued.