As you have all DIMM on each server is same you can replace only failed DIMM with new one. I assume new one also have of same size and speed of DIMM which is getting replaced.
We have 2 PE T710 servers that had a memory failure (ironically the same slot) B6 the first server has 2 cpu with 128 GB I got the replacement ram which they sent me 2 matched sticks. My first question is do I have to replace 2 memory modules and if so what is the matching pair to B6 the diagrams I have seen online says B3 but want to make sure. The answer to this question will answer my question for the other server as well.   Thanks
T710 Memory replacement issues