Is it ok to diet if im ill and cannot exercise?

i have been ill for some time resulting in me having little energy, being dizzy etc. unrelated to this i have to have ma gall bladder out and the doctor said he would like me to loose weight before doing so.mu mums friend had this operation and is much bigger than me and didn't need to diet and is also going to have and has had much riskier operations for which she again doesn't or didn't need to diet. as a result of my current condition i cannot exercise so wondered if there are any diet tips anyone could give me so i could loose weight without exercise. i have always thought you needed to combine the two to loose weight but the doctor knows i have no energy but still expects me to loose weight. he said he would book me in to see a dietitian (i was in hospital at the time) but i never saw one and was just told when being released to go on a low fat diet but that was it. any tips would be much appreciated thanks.