If you use the present tense, than you are asking where she works now. If you use the past tense, you are asking where she worked at some time in the past. If you ask with the past tense, then for the question to make sense you'd have to give some indication of when. It is possible that she could still be working there, though depending on context the other person might find it necessary to specify that.
Side note: "where" in this context could mean a geographical location or the identify of the company.
"Could you tell me where she works?" "At Murphy Motors". This is where she works now.
"Could you tell me where she worked before she was married?" "She worked at Murphy Motors. And she's still working there now." Or, "She worked at Murphy Motors, but now she works at Kelly's Accounting Services."
"Could you tell me where she worked in 1978?" "She worked in New York." As I said, could also be a geographical location.