Leo says Google Photos is a great place for this. He could share an album with anyone in a couple of different ways. There's 'read-only', where people can see the photo or click the sideways ellipses and download the image or movie. He could also set up an album that people can contribute to. Then at the event, he could tell people to submit photos to that album.
A lot of professional photographers use SmugMug. He could set up a special page with a password. Then, people could not only download photos, but SmugMug has a store. People could order a t-shirt, coffee mug, poster, and more.
He could also use Flickr for this, which offers 1TB of free photo storage.