He has to implicitly (whether or not he includes the latest update to that other unvalidated partition) when he builds his block at the end of the chain that includes that other unvalidated partition. Longest chain wins so he won't want to ignore that last block.
By building on top of another partition's block, he increases the risk that his block will be orphaned because he has cannot tell if it contains a double spend. For him it is better not to build on the best block at all, but to maintain his own partitioned chain of blocks.
The partitions are strictly independent. Remember that.
Okay monsterer really. I must end this now. I am 100% certain my point is unarguable.