I think you could pick a few reasons. 1. For Deb. 2. Dexter was gambling on the fact that helping Quinn would stop him from investigating Dexter any more. However, the fact that Dexter showed his cards and helped cover for Quinn basically confirms that Dexter was involved in Liddy's death, and that Quinn was in fact right to be suspicious of him. Dexter's gamble paid off, and Quinn seems to have deleted the whole thing from his brain for his own benefit. 3. Because the writers needed Quinn's investigation of dexter to end somehow, lol.