I'll try to answer by giving the perspective of someone who is likely in a similar situation as your friend.
If the other person is living in an English speaking country, and they are proficient in the language, then accept that it may be unrealistic for you to expect him to speak his native language.
I live in a country and use exclusively the language of that country rather than my native language and have been doing so for over three years. When forced to speak my native language, I actually have a difficult time as I am not used to speaking it, and it is sorta frustrating. I am fine with speaking it to people who can't speak the language of the country I am living in, but when someone proficient in the language tried to demand I speak my native language it kinda bothers me. If they want to speak my native language to me, that is their perogitive, but I will allow myself the liberty to reply in the language of my choice.
I would also like to add that I am happy to help someone learn by answering questions about the language itself, but there are other ways to get speaking practice than forcing me to speak the language.
EDIT: Since this was migrated from the workplace, I presume it is a work relationship. It would be especially upsetting to have to speak the native language in a work environment when the other person isn't more proficient in the native language than the language usually spoken at the company. It is presumably not in that person's job description to have to use that language, espicially not to teach others the language.