"I was helping him at work."

Translation: Pomagałam jemu w pracy.

[va-diim]

It marked incorrect "jemu pomagałam w pracy." Is it incorrect?

[Jellei]

As often - it is sometimes possible, but in very limited contexts... imagine this:

Jemu pomagałam w pracy, jej pomagałam w domu, a im pomagałam w ogrodku.

(Him - I was helping at work, her - I was helping in the house, and them - I was helping in the garden)

So I guess it's better to reject such answers, than to lead someone to an impression that this would be natural.

[va-diim]

That makes sense, thanks. Russian word order threw me off on this exercise.