Western instruments have been used for a long time, e.g. Harmonium, from the time of the dramas. When did they first use them in the talkies? I have read many articles making vague, general remarks. Recently, I found a concrete example in writing. Here is S D Burman reviewing the music of Chandidas (1931, Bangla) in 1931. An excerpt:
Of course, most people already know the great work of Karthik in chronicling the use of Western tunes by Indian film musicians: