Short story

He had sympathy with for/He was sympathetic to Helen, but he had a suspicion, that she was not as [easy]/straightforward/open as she wanted to seem. They had met a mounth ago then/when [?] Helen [got into] worked/started work in his office as an account manager. Since then they had become friends. He had a great belief in her.
Everything [went]/had gone wrong that morning. First, he found that the company's account [was]/had been frozen. Then Helen didn't come to the staff meeting at the appointed time. He [had a knowledge of]/knew her punctuallity. She couldn't [miss]/have missed the meeting without a very important reason. But, why [she] didn't she tell him//hadn't she [warned him]/let him know ?  He had a sudden yearning to call her. However her phone was off.

Past tenses describing reported events already in the past at the time of the narrative [ itself in the past ] should be in the past perfect tense :

'He had a great belief in her and had sympathy for her, but he had a suspicion . . [simple past].  They had met when . . . . everything had gone wrong . .' [past perfect]