same customer left a second 1 star review from a burner account

Screenshot both reviews now, with the profile names and dates showing, before you report anything. If the report gets turned down you'll want that record for the appeal, and it's what support will ask for if you escalate. The review management page in your Business Profile shows whether each report is still pending or was rejected, and lets you appeal from there. Most people report once, never check back, and assume Google ignored it. In your reply to the genuine review, just stick to what happened. Don't hint that you know about the second account.

Report it as fake or conflict of interest. You'll have better odds than usual since you can point to the first review, the wording or details probably give it away. Reply calmly to the genuine one and leave the second one unanswered while it's under review. Readers can usually tell.