I'm Sergei Tokmakov, California attorney (Bar #279869). Cofounder departure pre-cliff is exactly what the cliff is designed for. The legal answer: per your agreement, she gets nothing — the company has the right to repurchase unvested shares (typically at the original price, often $0.0001/share).
The practical answer: do a clean separation agreement that (1) confirms repurchase of unvested shares, (2) gets a full IP assignment for any work she did, (3) includes mutual release of claims, (4) optionally provides modest "goodwill" equity (capped at what she'd vest in 6 months) in exchange for advisory work going forward. The IP assignment alone is worth giving up some equity for — VC due diligence will flag any IP gaps.
I help founders with these separation/cleanup situations on a flat fee basis depending on complexity. Informational only.