Q: Invalid number

My guess is that when you entered the number of your friend in your contacts app, you have entered it in the wrong field.  Try deleting your friends contact details from the contacts app and then enter it again paying particular attention to the fields where you enter details and make sure you get it right

When you look at Settings > Phone > My Number, does it begin with the country code (+1 in the U.S.)?  It should.

Check Settings > General > Language & Region > Region and make sure it is set correctly.

what country, area code you using  for your friend and how many digits is the number after the area code?

So is the number in the form:

(530) 987-6543?   (With parentheses and hyphen).

so have you tried entering the number as "+1 530 XXXXXXX"?

Emphasizing you must enter the area code, even though it's the same as yours.

Did your friend used to have an iPhone?

If you can’t get text messages from an iPhone - Apple Support

Is this the only person you're having texting issues with?

Yes, your friend has an Android currently, but if he used to have an iPhone, the article applies.  He may need to de-register iMessage as explained there.

Yes, your friend has an Android currently, but if he used to have an iPhone, the article applies.  He may need to de-register iMessage as explained there.