Boundary nodes cannot collude to break the security guarantees that the IC provides. They act as a gateway to the IC and translate browser requests into API calls and route them to the replicas. See Boundary Node Roadmap for a description and roadmap.
Once a user has the NNS public key, all responses from the IC can be verified, like that integrity can be guaranteed. See Section 1.9 of https://internetcomputer.org/whitepaper.pdf for more details