Am I obligated to ask my friend for the musical instrument in order to destroy it, and am I sinful if my friend uses it?

We ask Allah to accept your repentance and make you steadfast. The sale of musical instruments and entertainment tools is not valid because they do not contain a legitimate benefit, and this is the view of the majority of scholars. You must retrieve this instrument from the buyer, even if by stratagem. If you are unable to do so, there is no blame upon you.