For a fasting person who declines food
For a fasting person who declines food: 1 supplication in Arabic with English translation and sources, from the Fortress of the Muslim (Hisn al-Muslim).
دعاء الصائم إذا حضر الطعام ولم يفطر
إِذَا دُعِيَ أَحَدُكُمْ فَلْيُجِبْ، فَإِنْ كَانَ صَائِماً فَلْيُصَلِّ، وَإِنْ كَانَ مُفْطِراً فَلْيَطْعَمْ، وَمَعْنَى فَلْيُصَلِّ أَيْ فَلْيَدْعُ.
If one of you is invited, let him accept; if he is fasting, let him pray (that is, supplicate for those present), and if he is not fasting, let him eat.
Fortress of the Muslim (Hisn al-Muslim), Sa‘id ibn ‘Ali al-Qahtani. MushafPlus translation made from the Arabic, not reviewed by a specialist.