Expected output of DFF_2 if DFF_1 has hold violation
If DFF_1 experiences a hold-time violation then the output of DFF_1 will be unknown for that clock cycle. The Q output from DFF_1 may resolve to a valid 1 or 0 but we can't determine precisely the minimum time required for that to happen. So, at the next clock edge the output of DFF_2 would become unknown. Note that if the input to DFF_1 does satisfy its setup and hold requirements at the next clock edge, then the output of DFF_1 would have a known value at the same time that DFF_2 would have an unknown value.