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If the person had a floating heater supply then, sure, the heater supply must be grounded. Or, more specifically, referenced to some voltage in the amp. Otherwise, with no reference, the heater to cathode voltage can, and will, be "anything" since it's unspecified. In particular, it'll float around till heater-cathode leakage current finds a path somewhere. That's where the hum comes from and it's not all that great for the heater insulation either.
Sometimes hum can be reduced by referencing the heater supply to a voltage more positive than the cathode. In other cases it's necessary to elevate the heaters even more to keep heater-cathode voltage within spec, which is a solid clue that the heaters must be referenced to the amp's supply because, if not, what's the heater-cathode voltage? The most common reference is ground.
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