I don’t know the answer to this. This is a question for Relevanssi support. Also, I’m not sure that it will work because the user field only stores the user ID in the database, so Relevanssi will need to have something that will let you map ID’s to Name. I did find this when I did a search that appears to answer this question https://wordpress.org/support/topic/using-relevanssi-with-advanced-custom-fields
