Grace knew that Heinz wouldn't have done this in advance without a reason. He was comforting himself.
He was afraid that he would regret it in the future.
Grace knew that she should not act out of her anger at that moment.
Her rationality was telling her that she should not be swept away by emotions. Instead, she must think about her grandmother's soul in heaven.
Grace was growing slightly impatient when she did not get a reply from Heinz after a long silence. "Do you mean that? Asking me to agree to his request."
"I only feel that this may be your grandmother's wishes," Heinz replied. "I am telling you this because I am not sure about that."
He had actually thought that it would be better to tell Grace the truth after careful consideration.
If he concealed it, Grace may only feel regrets when she would learn about it in the future.