I'm not really up to speed on AB (AB hater), but I have done some ab work way way back and not too recently did try to interface with AB using EthernetIP. Not sure about Device Net but I thought it was way different than EthernetIP. In fact I believe it is an entirely different physical connection than Ethernet. You stated that your clients bought Ethernet IP cards for the Eaton, well, welcome to my world. If the client decides on anything it will be AB regardless of what you ask them to do.... The ModbusTCP would have been the way to go and if they had bought ABB drives, Modbus RTU is standard and I have done a number of ABB modbus systems, and they work perfect.
The problem with EthernetIP is that the drives never have EnetIP as a master, only as slaves and Opto22 has only a slave available which does work great, is well documented and has a configurator download for free to set up any of the Pac brains to connect with EnetIP.
The other problem with EnetIP is that all your switches and routers have to have IGMP snooping or the EnetIP will bring your network to a halt with useless traffic.
I suggest having the client send the EnetIP cards back and get what you asked for or I believe that Eaton also has ModbusRTU as a standard protocol and you could connect using a serial module or a controller port. It's also likely that the EnetIP card is connected to the port same that has ModbusRTU. If the distance is not too far <50 feet, and you have an R1/R2 you can use the RS232 port on the controller. If you have an S1/2 you can use the RS485 ports to connect. One more possibility is and this is not very sanitary, but you could buy a AB micrologix 1400 and use it as a translator.
The only thing common about CIP is if you are using AB... It would be nice if Opto could develop a EnetIP Master, I suspect the royalties and the complexity is ridiculous.
When I write a proposal and sign a contract, I require they provide what I spec, otherwise, you're at their mercy.