Friend bought Android phone, but my iPhone keeps trying to send them iMessages. Any fix?
You can ask your friend to Deregister iMessage. It can be done both on the iPhone or if they no longer have access to the iPhone, on the Web.

See the linked Apple Support article to learn more:


Deregister iMessage on your iPhone or online


Since your friend has already moved to an Android phone, it would be easier for them to deregister online. Here's the webpage that can be used to deregister iMessage:


Deregister and Turn Off iMessage - Apple Support