since this forum is free all tips are exactly just that (and not legal binding advice).
Loans are an important tool, not the least for the reasons the hedgehog mentioned above. Even when executed in a non-optimal way. Googling legal stuff with partial knowledge is a dangerous thing. People turn to this forum for advice. Your overconfident statement is misleading them.
A “zinsloses darlehen” without expiry date becomes her money after 10 years
is incorrect and not covered by your quoted exerpt on the site of my former teacher. Even after the statute of limitations hits, it’s still your money. You can sue for it, it can be compensated with money she owns you or somebody else. Taking it yourself from her (in a non-stoopid way) would not be theft, as it’s not somebody else’s property. When sued, she would have to explicitely rise the statute of limitations, otherwise she loses. Moreover, putting stuff in your tax declaration as debt (to reduce her tax load a bit) means she recognized the debt. Which triggers the 10-year-timeframe anew.
The real risk is her ability to pay you back.