Should I be a suspicious wife?

I know a woman from my in-laws side. Both are in fifties. wife is suspecting husband for his clandestine affair with a woman who is the deceased wife of his brother ( cousin ).we know that he and wife of the deceased brother are honest and do not indulge in such adultery.

But she is not convincing. She checks his phone calls account and if there is any expenditure on calls, she suspects that he had talked to the other woman only.

She is making the life to this man a hell. How to convince her is a million dollar question to us.

Please advise.