After a brief, slightly awkward conversation with her never-before-mentioned old classmate Lindsay, Rory sees her kissing Dean, and realises her ex-boyfriend has moved on. Lane goes to school with Dean and Lindsay, but she has apparently not noticed that they are now together. She addresses this by saying that she “failed” Rory by not keeping tabs on Dean for her.
Lane did used to do a good job letting Rory know what was happening with Dean, but that was back in the days when Rory and Dean were dating. I guess we can assume that Lane is now busy with her own boyfriend, and possibly the relationship between Dean and Lindsay is still quite new.