Theme updating from invalid theme source

I inherited a theme with really old code and no documentation. I got most of the code stabilised (still learning wordpress) and upgraded the plugins, core etc (all over 5 years old).

However the theme keeps saying it needs to update.
After letting it update the theme is totally changed.

The Original code had a theme called Blank.
The theme also had a child theme called blank.
Child theme is active.

The sites name is [ redundant link removed ]

When the update theme button is pushed the theme updates to a theme called festive (same name as the site). The updated themes author, sources, look etc are totally not anything to do with the blank theme.

Some how the theme update code is using the theme called Festive to update the theme called blank.

1 – where is the code to update a theme held?
2 – I know you can add a filter to stop themes updating (but not where to put that code without making a muPlugin) but there must be a way to unhook the theme called festive from updating my clients site.

any help appreciated.

The page I need help with: [log in to see the link]