"When did she tell him that?"
Translation:Kdy mu to řekla?
Hi. You have used the correct words, but the word order is incorrect. The pronoun "jemu" is only used when it's emphasized (ie. at the beginning or at the end of a sentence). Since you've not emphasized it, you have to use the short form "mu" and this has to follow the rules for clitics (as it is a clitic). There are two clitics in the sentence ("mu" and "to") and as you might know dative precedes accusative in the order of clitics.
You start with the question word (Kdy), then clitics follow - dative first (mu), accusative second (to) and finally the full verb (řekla). It's a neutral question.
To emphasize, you can modify the word order, but stick to the rules I've described.
Kdy to řekla jemu? / Jemu to řekla kdy? / To mu řekla kdy? Řekla mu to kdy?
The "to" is a clitic, so it has to follow the 2nd position rule.
In the main translation, there is the second clitic "mu", which has higher priority as a clitic than "to" and therefore is placed second. Then "to" is sticking to the first available spot after the 2nd position.
In your sentence, "jemu" is not a clitic, which makes "to" the only clitic in a sentence and following the 2nd position rule it will be 2nd in the sentence. "Kdy to řekla jemu?"