One of my friend came to my place for a cup of tea, i introduced her to my mom. After she left my mom said she is a nice girl. When we met in the college the other day, i told her that my mom liked her very much and in reply she said "tell her thanks" I really want to know if telling someone thanks is a correct sentence?
"Thanks" is informal/colloquial; the "correct" English is "thank you".
That said, "Tell her thank you" is commonly accepted as correct English; it is a shortened form of "Tell her I say thank you" (which is also correct).