Single word for "the one being stolen from"?

Solution 1:

There is no such single noun that I am aware of. We might say the one stolen from or the victim.

Colloquially you could find terms like the mark or the patsy but I would not recommend using them except under restricted circumstances and informal usage.

Solution 2:

Well, you could say "the robbed".

Some of those people who were robbed by the banking system in 2008 are still having a hard time, eleven years later.

Solution 3:

Theft is a crime so legal words are appropriate. The legal word for the victim of theft (or robbery or burglary) is (in England) "the loser".

See https://www.sentencingcouncil.org.uk/wp-content/uploads/Theft-offences-definitive-guideline-Web.pdf