Oii... I like how after he says he only has cards to work with, everyone gives ideas other than cards.
Also, his performance is really soon. Why would you suggest something completely new to perform in two weeks time? We always get upset about people buying a new trick, practicing for a short time, putting up a mediocre video on youtube, and exposing the effect. Yet, you want him to do this exact same thing for a live performance? That makes zero sense.
Back to the OP. If you have already agreed to do this 4-5 min performance, then make sure whatever you choose to do, it is something that you have complete confidence in doing. You should choose something that you have already been doing for a long while. However, just kind of tweak it a bit to make it play bigger with different patter. I will name some card plots that can play really big, and if there is a plot that you already do from the list, then great =).
Invisible Deck- Can Play HUGE for crowds. Have 1 person pick the color, 1 person pick the suit, 1 person pick face card or numbered card, last person pick the value! This makes it so many people are invested in the trick, and not just one. Put the deck of cards on a table and note to the crowd that you will stand 5 feet away from the deck at all times until the reveal. ETC...
Cards Across (mentioned above) - Watch some Bill Malone footage of this for patter ideas! At the minimum, you will have two people invested, but could even invited a third person to do the card counting.
FUSILADE, or another multiple selection routine - It is a basic "choose a card and I will find it", but with upwards of 15 cards from 15 different people! The fun part is if you can remember each person's name along with their card!
Forced card and reveal in a big way: Idea for the reveal could be Watermark, Bruise, Ash on arm, Plant a matching jumbo card under one of he chairs, etc...
etc... etc...
The things you want to focus on are, pulling the crowd in, making it visual, have a clear ending. One trick is all you need. Just make sure it is a trick that you already know how to do. Do not go learn a new trick for a performance two weeks away.
People did however give good advice on tricks that work well for the stage, and you should definitely look into learning those. Just do not plan on doing it for a performance two weeks away.