Theresa, the elderly maid from La Vallée, definitely got the short end of the stick. When Madame and Monsieur St. Aubert pass away and Emily takes off, no one's looking after poor Theresa. Where's her retirement fund when she needs a little pick-me-up? But when it comes down to it, she's genuinely sad about the death of her boss: "A-well-a-day! The news almost broke my heart, when it did come" (1.8.35).
Emily writes to Quesnel about Theresa because she's not too sure the old servant can cope on her own. Turns out she doesn't have to worry, since Valancourt is shelling out the big bucks for Theresa to live comfortably. Theresa's got a soft spot for the guy who spots her cash, as she makes abundantly clear to Emily. If Theresa can put in a good word for Valancourt, she'll do it in a flash.