I have dial-up and have no problems with Spybot, or anything else, because of it. There's no reason why Spybot shouldn't work perfectly well for your friend. It's a 9.27MB download, so it's best to use a download manager to get it so that, in the event of the connection being dropped for any reason, the download can be resumed from where it left off upon reconnection.
After updating Adobe Reader and QuickTime recently, 22.4MB and 22.3MB respectively, 9.27MB seems quite small. The 2 large downloads gave me a couple of good opportunities to catch up on my reading!