Lilly, here are things I have tried here.
If the other manager is responsive, discuss the evidence in support of each conflicting data item and let the evidence (sources) tell which is most accurate. If the other manager is non-responsive, or asserts facts in the data fields with no sources (LNAB, birth-marriage-death dates), ask for help from a mentor to do the merge. After all else fails, there is a Mentor Intervention Request (MIR) process that may help.