Close contact
The reasons why Bai Zemin decided to help Shangguan Bing Xue were varied.
One of the reasons was because she still had his Bronze Bell in her hands. The Bronze Bell was a very important Magic grade Treasure for Bai Zemin as it was currently his safest defense and he could not afford to lose such an item casually as he did not know if he would be able to obtain another one anytime soon.
In fact, the only reason he lent the Bronze Bell to Shangguan Bing Xue at that time was because his breakthrough to First Order and the acquisition of his Blood Berserker job depended largely on whether she succeeded in pinpointing the location of a First Order creature.
That way, he would be able to prepare his strongest attack from a distance great enough for his killing intent to be sensed by his enemy or he might end up failing miserably... and if he failed even he would have to flee poorly or be killed by the army of First Order creatures.