Fields throw away edits when updated - there's little you can do to change this. There is a * MergeFormat switch that can be added to field codes in order to retain formatting. The major drawback, however, is that it "remembers" formatting by character location. So if the text changes (gets shorter or longer) the formatting "goes crazy". Therefore, I don't think that will help you.
So all you can really do is re-apply the formatting after each TOC update. The fastest way to do that is using code (a macro, for example).
Here's a macro that works in a quick test on my system. It searches for each TAB character in the TableOfContents. Then, based on the additional information from the OP that only the second tab should be recognized, uses a second range to check whether the position AFTER the tab is within one centimeter of the right margin.
If that's the case, it repositions the find-target Range from that point back towards the beginning of the paragraph, then applies the formatting.
Note 1: You can change the "cushion" used to determine the distance to the right margin by changing the formula assigned to PageNumLimit.
Note 2: I defined a STYLE named Toc1_Text and use that, rather than formatting with BOLD directly. Doesn't really matter, but it feels "cleaner" to me than direct formatting :-)
Sub FormatTextInTOC()
Dim rngFind As word.Range, rngFound As word.Range
Dim bFound As Boolean
Dim toc As word.TableOfContents
Dim infoH As Double, pageNumLimit As Double
Set toc = ActiveDocument.TablesOfContents(1)
toc.Update
Set rngFind = toc.Range
pageNumLimit = rngFind.Sections(1).PageSetup.RightMargin _
- CentimetersToPoints(1)
With rngFind.Find
.ClearFormatting
.Text = vbTab
.Style = word.WdBuiltinStyle.wdStyleTOC1
Do
bFound = .Execute
If bFound Then
Set rngFound = rngFind.Duplicate
'rngFound.Select
rngFound.Collapse wdCollapseEnd
infoH = rngFound.Information(_
wdHorizontalPositionRelativeToTextBoundary)
If infoH >= pageNumLimit Then
'rngFind.Select
rngFind.Collapse wdCollapseStart
rngFind.MoveStart wdParagraph, -1
rngFind.Style = "Toc1_Text"
rngFind.Start = rngFind.Paragraphs(1).Range.End
'rngFind.Select
End If
End If
Loop While bFound
End With
End Sub