The most popular blessing expression we use during the Mid-Autumn Festival is 
“中秋节快乐！(Zhōng qiū jié kuàilè.) Happy Mid-Autumn Festival.”
If you and your friend are far away from each other, you can use two beautiful sen-tences from an ancient classical Chinese poem:
Dànyuàn rén cháng jiǔ, qiǎn lǐ gong chánjuān.
但愿人长久，千里共婵娟。
May we all be blessed with longevity. Though far apart, we are still able to share the beauty of moonlight.
Hài shàng shēng míng yuè, tiānyá gong cǐ shí.
海上升明月，天涯共此时。
The silver moon rises steadily above the sea; we, hither and thither, share the moment together.