My iPhone can't text friend's Android, but my wife's iPhone can

Solution 1:

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.

Solution 2:

You mentioned in a comment that the friend uses an iPad. Therefore if you send him an iMessage, it’s delivered to his iPad and not the Android phone. Settings > Messages > Send as SMS only applies when the iMessage cannot be delivered but it is not the case when it’s successfully delivered to the iPad.

I can imagine two solutions:

1. Your friend must turn off his iPad (airplane mode is enough) when he cannot check it.
2. He must deregister iMessage for his phone number, keeping only e-mail address as the iMessage ID. Then start a new conversation with him on your iPhone. Type his name to the To: field and select the item with the green phone number. This selection works for my friend having iPad and a “dumb” phone. However, I haven’t tested the procedure of deregistering iMessage.