Rhonda is Alex's fan
Rhonda: Could I get a picture too?
Alex: Only if you're single.
Rhonda: I've been married 16 years, but nothing's written in stone.
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This is a reference to an event in the bible (Exodus, 31:18) where God presents Moses with the ten commandments, written in tablets of stone.
The ten commandments are a fundamental part of the rules of Judaism, so if you say something is written in tablets of stone, it means that it's a rule that cannot be changed.
"Nothing's written in [tablets of] stone" means that the rules can be changed.