Surely it's an FI.
If you could be given access then you would be able to find out whether it's safe and compliant, without that you can't check a whole bunch of important things so it may or may not be safe. There could be a C1 at the DB for all you know.
Not sure about the regs, but I don't think it is necessarily a critical issue that the tenant can't access the DB. You could argue on the grounds that the lights could trip leaving them in darkness but it's not necessarily electrically unsafe as a result. If you can test it and it's all kosher then I'd say a C3 on access to switchgear of critical circuits or something like that would be fair. Maybe a time delay RCD with a normal 30mA fused RCD spur on the tenants side could be a minimum cost solution?
Personally I would still create the EICR and report on anything could could still safely isolate, citing an operational limitation for no access to DB or supply equipment. That way you can still justify the charge for your wasted time!