Best thing to get

Go to GuitarCenter.com and look for "portable amps" or "guitar amplifier" for ideas.

And use a 1/8" to 1/4" jack cable to connect PC headphone out to amp input. 

I don’t know if it makes a difference but he’s not on a guitar, he just needs it to play the backing tracks. He’s dancing to the music. I don’t know now having looked but what’s the difference between the amp and the speaker? Sorry for the total ignorance. He’s currently using a vocal star multi media speaker that he hooks up to his laptop. 

Try Musician’s Friend for something bigger than what he’s using: https://www.musiciansfriend.com/karaoke      Here’s an option, from Best Buy: https://www.bestbuy.com/site/lg-xboom-550w-speaker-system-black/6297127.p?skuId=6297127    Many of the newer boom-boxes have both Bluetooth and USB, which will make laptop connection easy.

This is why you need to give ALL the info upfront. If he’s already using a Vocal Star (VS) and it’s not loud enough, he just needs a louder unit.
Is he using effects or other specific settings on the VS that he needs on a better unit? If so, please specify. If not, then a better amp/speaker combination unit as mentioned above will work.
This will blow him away in loudness compared to the, dare I say it, toy Vocal Star.
https://www.musiciansfriend.com/amplifiers-effects/fender-champion-100-guitar-combo-amp/h97919000001000

Thank you for your help and advise. Sorry for not giving all the nap in the first instance. I’m pretty new to this and am completely clueless. They’re just kids wanting to give this a go so all your tips and links have been invaluable to help me figure this out to help them. Thanks again 

Feel free to ask more questions on this thread. Good luck.

Main problem is this stuff is so specialized no one can advise without knowing an incredible amount of details. Like, how big a space? How many people? How loud? How compact/easily transportable? Does it have to look good or just sound good? Or just sound loud? What's the budget? 

The amp provides the power, the speaker makes the sound. Often times speaker and amp together are referred to as the amp. 

Options range from a $20 wireless speaker barely louder than a Bose radio to the AC/DC road tour rig. So it helps if you can narrow it down some.

Look for a music store in your area, Talking to someone face to face or mask to Mask is a good starting point.Check out other buskers and ask them for a few tips.I'm with millercarbon on this one it raises a multitude of possibilities.Did I just say I'm with millercarbon?

"Did I just say I'm with millercarbon?"

You stole my words. This day will go down in history.

I never heard of busking until now. I looked it up and it said performing for donations. Is he dancing for donations?

My wife and her friend like dance while I pump iron in basement. She always having girlfriend over to practice routine and end up watching me too. Maggie makes good speaker for classical music, not disco text. Good luck you pursuit.