Old Man Lightning contacted Gravis and told him all about their situation. He didn't keep what he thought about Gravis' action secret. After all, Gravis had to understand that he was different from other Lightning Sect cultivators. Even if he felt isolated because of that, he needed to understand the full range of the issues.
Old Man Lightning had heard about Gravis' past in great detail, so he knew that Gravis felt alone nearly all the time. He had just found a place to belong to, yet Old Man Lightning was about to destroy that again. Old Man Lightning actually felt a little bad about telling Gravis all these things.
"Why didn't you tell me earlier?" Gravis asked in confusion.
Old Man Lightning sighed. "I didn't want to make you feel isolated again. I'm sorry," he said with sympathy.