Well the important question is does he have a choice weather he can to go or not? If he is being forced to go then its a bit unreasonable for him to stomach the cost. I do agree with you that every man should own at least one Tuxedo but the fact is that majority of the people today even in those industries don't. I have several friends that work at client facing roles in the big banks here in NYC some wear suits everyday and get invited to these things regularly but even they don't own a tux and usually just wear a black suit with a black tie to those things.
Whether or not he has a "choice" is debatable. It sounds like one of those things that he could back out of if he wanted, but it could potentially hurt his long-term career prospects if he did.