If a person finds it difficult to fast, or to perform Prayer while standing due to illness, and wants to consult a doctor in this regard, he may consult an honest Muslim doctor. If there is no Muslim doctor, he may then consult a non-Muslim doctor that is trustworthy.
The late prominent Saudi scholar, Sheikh Ibn `Uthaymeen (may Allah have mercy upon him) said: Some scholars are of the opinion that it is the honesty of the doctor that is the condition to consider in this respect, regardless of his/her religion. They said that since the doctor is honest, the Muslim can depend on the doctor’s judgments even if he/she is non-Muslim.

Those scholars give as evidence for their opinion, that the Prophet hired an honest non-Muslim from the tribe of Ad-Dayl called `Abdullah ibn Urayqet to guide him and his Companion Abu Bakr on the way to Al-Madinah during the Hijrah. At that time, it was dangerous to count on a disbeliever because the Quraish were pursuing the Prophet (peace and blessings be upon him) and his Companion, offering two hundred camels as a reward to whoever seized the Prophet and Abu Bakr and brought them back to Makkah.
Besides, it is known that there are non-Muslim doctors who are more honest and efficient than some Muslim doctors, not out of being mindful to Almighty Allah or seeking His rewards, but out of their keen interest in preserving their reputation and honor.
Based on the above, if a non-Muslim doctor who is honest and efficient tells a Muslim patient that it is harmful to his/her health to stand during Prayer and that he/she should, instead, pray while [sitting or] lying down, the Muslim can follow the doctor’s guidance. This is also the case if a non-Muslim doctor tells the Muslim patient that fasting is harmful to his/her health or may delay recovery. This is the stronger point of view of the issue in hand.