Say Person A (Adam) owns the domain example.com, but is allowing free use of the domain name to his friend, Person B (Bill). As the owner of the domain, Adam logs into his registrar and points the nameservers to Bill's host, GoDaddy. Bill hosts example.com on GoDaddy and wants to use G Suite and Gmail. Bill then configures the MX records in the GoDaddy control panel to point to Gmail (e.g., ASPMX.L.GOOGLE.COM, etc), and everything works.
My question relates to security of Bill's email: As Adam owns the domain name and is the only one with control of the registrar control panel, can Adam do anything from the registrar level to ultimately view Bill's Gmail messages? Or, perform a sequence of steps to reroute, or make copies of Bill's mail without their knowledge?