Is it wrong for a married man and a married woman to be alone with each other?

My husband had to go on a work trip with a female co worker. They had dinner and drinks together, went on a ferry boat together, a national park. Etc. But my husband said he would never cheat on me. This woman is old, ugly and fat. Maybe not to everyone but to me she is.
Is it wrong for a married man and a married woman to be alone with each other?
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