I think what truly made the difference for Diana, at the time Mannakee was working as her RPO, is that he was actually *there*. It doesn't really matter what Barry's sentiments towards Diana were at the time. What mattered was how important Barry was to Diana and what Diana saw in him.
She was facing the hard, cold reality that her fairy tale marriage was not as she expected it to be with Charles gone a lot of the time without her, still pursuing his own interests that Diana had no liking for and frankly, Charles did not have the time to play the ever attentive husband to his wife.
Mannakee was on the job and that meant wherever Diana went, Barry went too. They communicated. They spent time together and formed a bond. What exactly that bond is we'll never know but for the sake of erring on the side of caution, its very possible Mannakee saw the relationship as protector/charge and Diana saw it as a new best friend and found in him the attention she desperately wanted from Charles.
Sometimes, I believe that absence makes the heart go wander and this could be precisely what happened with Mannakee. He was there day in and day out and Charles wasn't. Its not hard to believe that an attachment would be formed with someone that's always around even though it was his job and his duty to be there.