Yes, with consent. "I won a meet-and-greet with @artist and it was amazing!" on a fan's own feed is theirs to post. Reposting it to your feed is usually fine if you credit them. Pulling a quote into a paid ad is a different matter — you need affirmative consent for that, because the fan's name and likeness become part of a commercial endorsement.
US FTC endorsement guidelines apply to influencers too: if the fan received something of value (the prize), that needs to be disclosed when the testimonial is used in ad copy. Meet Your Fan provides a standard #ad / #contestwinner disclosure snippet in the winner-announcement tools.
Video testimonials add an extra layer — you're using the fan's voice and face. Get it in writing, keep a copy, and don't edit the testimonial in a way that changes its meaning.