All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
We have already published Fatwa 83282 clarifying the religious view in general about obesity and we mentioned some guidelines in this regard, so please refer to the above Fatwa.
As regards the supplication, she may supplicate with what she wishes of the supplications that mean what she wants, and she may supplicate with general supplications that are proven authentic, like the supplication of a person in distress. The Prophet said: "Allaahumma rahmataka arjo fala takilni ila nafsi tarfata 'ayn, wa aslih li sha'ni kullahu, la ilaaha illa ant." "O Allaah, it is Your Mercy that I hope for, so do not leave me in charge of my affairs even for a blink of an eye and rectify for me all of my affairs. None has the right to be worshipped except You." [Ahmad and Abu Daawood]
You may also supplicate with the supplication of the Prophet Yoonus (Jonah) when he was in the belly of the whale. The Prophet said: "The Dua' (supplication) of Thun-Noon (Jonah) when he made this supplication when he was in the belly of the whale: “None has the right to be worshipped but You (Allaah). Truly, I have been of the wrong-doers.” No Muslim man supplicates with this supplication except that Allaah answers his supplication." [Ahmad and At-Tirmithi] There is no difference here between a man and a woman, it applies – to any Muslim person.
On the other hand, this sister should consult trustworthy and experienced doctors and she should seek the Help of Allaah and not despair. Abu Hurayrah narrated that the Prophet said: “A strong believer is better and is more beloved to Allaah than a weak believer, and there is good in everyone, (but) cherish that which gives you benefit (in the Hereafter) and seek help from Allaah and do not despair or be lazy.” [Muslim]
Allaah Knows best.