Here are simple, respectful ways you can say something to Marwadi Śvetāmbara Jains when they are fasting:

• जय जिनेन्द्र (Jai Jinendra) — the standard Jain greeting.
• आपका व्रत मंगलमय हो! — May your fast be auspicious.
• आपकी तपस्या सफल होवे — May your penance/tapasya be successful.
• आपणी साधना ने शुद्ध राखो — May your practice remain pure.
• व्रत के समय आपकी strength और स्वास्थ्य बने रहे — May you stay strong and well during the fast.

Notes:

• Śvetāmbara and Digambara traditions both use Jai Jinendra as a common greeting; these phrases align with the Jain emphasis on purity of vows and earnest spiritual effort.
• If you know a specific occasion (for example, a vrata like upvas, ekadashi, etc.), you can tailor the wish: “आपणो उपवास सफल होवे” (May your upvas be successful).

If you’d like, tell me the exact occasion or the language you prefer (Hindi, Marwari, or standard Gujarati/Hindi), and I can tailor a few more precise phrases.