Translation:She talks about him only when she is with us.
Interesting, because there's a second version of this one: Elle ne parle que de lui quand elle est avec nous.. DL translates that as "She only talks about him when she is with us."
So it looks like they'll accept the same English version for both, but we must be cautious when translating into French regarding the position of the "que"! There's another "pair" of exercises about a guy only watching romantic films on Mondays which has the same pattern, but I don't know if they'll accept the same English for them!
'she talks about him only when she is with us' is accepted. But as others suggest, the sentence is ambiguous in english. This is because, unlike french, english uses verbal stress to establish one or other nuanced meaning while being flexible about the placement of the adverb 'only'. [The czech-english duo pages also see this issue.]
BTW, there is no need to teach french by translation - that is the Duo way, but immersion is arguably better.