Ask HN: Calendar Scraping/Sync Tools?

Hi HN. Long story short: my girlfriend is a massage therapist being squeezed for big commissions by a marketing service (treatwell.com) that has shut down its API for bi-directionally syncing its calendar with hers, probably as a deliberate lock-in tactic. Treatwell reads her google calendar to avoid clashes, but writes only to its own when someone books through treatwell. With her clientele increasing through independent referrals, she's investigating alternatives including self-hosting, but assuming it's not worth ditching treatwell entirely, what tools would you recommend for scraping her treatwell calendar to keep her google calendar in sync?