When creating/editing a tiddler, the only difference between the default (text/vnd.tiddlywiki) and HTML (text/html) interface are the editor toolbar buttons, nearly all of which are applicable only for native TiddlyWiki wikitext parsing anyway.
Since your client only intends to use HTML syntax, I suggest completely disabling the editor toolbar (see $:/ControlPanel tiddler, Settings tab, Editor Toolbar section, "show editor toolbar" checkbox).
Then, if you leave the "type" field blank, the content rendering will default to using the "text/vnd.tiddlywiki" wikitext parser for which, as Jeremy has already noted, most HTML elements are interpreted correctly
Thus, your client can completely ignore the content type field value and won't ever need to switch it before saving a tiddler.
The only potential issue would be if the client inadvertently uses wikitext formatting syntax, such as doubled slashes for //italics// or doubled single-quotes for ''bold''.
On the other hand, a benefit of using the default wikitext parser is that links to other tiddlers can be easily entered using doubled square brackets [[like this]].
-e