My iPhone can't text friend's Android, but my wife's iPhone can
This occurs because Apple still believes the phone number is using iPhone, but it can vary from device to device whether this impacts sending and receiving messages to that number (and how long it takes for that to resolve).

The quickest way to resolve it is for your friend to read the Apple support article Deregister iMessage. The deregister process can be done from the old iPhone (if it's still available) or online. It still may take a few hours for all Apple devices to recognize the number no longer uses iMessage, but it could take much longer without completing the deregister process.