There is a specific method of gluing, & inserting the stones so that they are: perfectly even, with: no glue showing.
The glue needs to be non yellowing, & made for glass to metals.
The method, as explained to me, involves holding the stone from the face of the stone, not the sides, as this always produces a crooked set-in. The stone is picked up with a toothpick or other thin instrument, from a dot of glue or other material placed on the face of the stone.
The stones need to be the same in color/lustre/size/faceting.
With this basic information, your favorite local jeweler, (or someone here at ebay?), may share the finer details with you.
As a Collector, my opinion is: I want to know if a button has been repaired.
There are times I have bought a repaired button, but it was MY choice to buy the repaired item.
When I have received a button with an unstated repair, (from someone who knows better), it ticks me off... a slow burn when my polite inquiry gets a weasely reply. A total write-off if followed by a request for feedback:(
Chris