Sorry - posted a comment instead of an answer. I agree with the first comment; using "was" there is correct. These may also work: The pilot saw one of the men had been taking photos. or... The pilot noticed one of the men was taking photos.

Yes, using "was" is correct. The singular form is necessary.

I agree with the first comment; using "was" there is correct. These may also work: The pilot saw one of the men had been taking photos. or... The pilot noticed one of the men was taking photos.