She's asking Jay because she is close to Angela. Why would the person who is the witch and is scheming tip her hand to the Empress? It's better to obtain proof and witness before confronting the culprit. Who would know better than anyone if Angela is up to something? Jay has a good reputation for being pure of heart, kind, and wise. Even if they are close, someone with Jay's reputation would want to help the Empress against the evil plot of her "friend".
She is also asking because Jay is close to the opposing prince. She may suspect that Jay caused her son to fall for Angela to ruin him so that he would no longer be the crown prince. It would be a great scheme if Jay is shooting for the crown princess title and is trying to make her chosen prince the next Emperor. It seems like the Empress is fishing to see where Jayna really stands in all this mess to assess if the girl will be an asset or an enemy to her.
How TF does the empress suspect Jay if she literally is not (entirely) involved with the mess she calls her son? Plus why is she asking Jay when she knows she's "close" to the Angela and it would more sense to suspect Angela since she is benefiting the most