His son is taking a $3,000 trip instead of moving out. What should he do?

“Our 23 year old son lives with his girlfriend in our spare mobile home. We charge him $300 a month for rent so he can save up for his own place.
They are taking a week-long vacation to Hawaii that will probably cost about $3,000.
I am absolutely enraged because he’s throwing away his savings. My wife thinks he should enjoy himself and spend his money how he wants.”