I've worked it out myself.
By using the 'disableHtmlEncode' attribute, HTML can be added directly.
Along the lines of the following
<e-treegrid-column field="Conformance" disableHtmlEncode="false"
headerText="<span data-bs-toggle='tooltip' data-bs-placement='bottom' title='MRO defines what information is mandatory, required or optional in the standard from the user.'>MRO</span>" textAlign=Center width="40px"></e-treegrid-column>