ReHIPS itself monitors mostly process execution and some related stuff (like hashes of files being executed, parent-child relation, process command line). Additional monitoring includes file system and registry access to block some locations. When programs are executed in isolation, most of security is handled by certified Windows security subsystem.
So processes inside one isolated environment are free to do as they please, inject in other processes running in the same isolated environment, etc. But only inside this isolated environment. Processes in other isolated environments or non-isolated processes, including system processes are safe.