I am puzzled that there is a single channel input in the robot called "ESTOP".
Other robots I have seen my collegues work with have any number of interlocks, so it sounds odd that there is just that one input and it is called "ESTOP".
Back to the question regarding connecting the safety part of the S7 PLC with the AB PLC.
It is not possible.
It is not even possible to connect the normal S7 program with the AB PLC.
The two PLCs do not support the same protocols, regardless of that they have ethernet ports.
There are a number of ways of connecting them together via Profibus or DF1, but for just one interlock signal I think you should just use a regular interlock relay.
And when you do so, I would suggest that you do it with 2 channels, and that via a contactor with forcibly guided contacts (horrible translation of "zwangsführten kontakten").
edit: To be precise, the contactor should be controlled by contacts in the safety relay or safety PLC. Not via a regular output in one of the PLCs.