"I was helping him at work."
Translation:Pomagałam jemu w pracy.
"Jemu" is just a stressed form of "mu", so both should be accepted, unless you put the pronoun at the beginning of sentence and maybe at the end(the first one is a rule in Polish for sure, the second just feels wrong, but I'm not sure there is really a rule against it, or is that just the convention).
I think the end of sentence works only for short sentences, because here putting jemu/mu at the end is makes it stressed, and thus jemu is necessary.
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.