There are two parents who live apart in this situation. Parent A and Parent B. One parent is RP other one obviously isn't but child spends 2/3 days (depending on it being term time or not) with NRP.
Child is found to have nits at parent A's House after spending 7 days there (not the usual but parent B had previous 7 days as it's school holidays). At drop off parent A says child has nits and parent B needs to treat them. At parent B's House there are 2 other smaller children who definitely do not have nits. At parents A's House there is one other child who also has them.
AIBU to think parent A should keep the child at their house until they have been treated as other child in the house will need treated as well and as it's not in parent B's house but they would risk passing them on to Parent B's other children?
So YANBU = Parent A should treat.
YABU = Parent B should treat.