Deleting files from the disk (even if you empty the Recycle Bin afterwards) doesn't really remove the data completely. The data will still be on disk, it is just that the index to where it is on the disk will have been removed, thus making it appear to be gone. Browsing in 'My Computer' / Explorer won't show the files, but a determined / knowledgeable geek could easily get them back with programs freely available from 'teh interwebz'.
If you are really concerned about privacy, tbrguy's suggestion to remove the disk is a good one. If you can spare the time, stay with the laptop whilst they replace the keyboard - it should take no more than 15 minutes to do so and carry out an operational check afterwards.