السلام عليكم ورحمة الله وبركاته My wife couldn't fast this Ramadan due to severe sickness & paid fidya. She wants to make up the fasts during shorter winter days. How should we proceed? If she tries but experiences extreme exhaustion and nausea again, should she stop fasting or continue?

Since her illness was temporary and she has now recovered, the fasts are made up (qada), and the shorter winter days are a good time for that. Fidya is for the one who can never make up, such as a chronic illness with no hope of recovery or old age, so paying it does not lift the making up in her case. As for while she is making them up, if fasting a day genuinely harms her health or brings real exhaustion and nausea, she may break that day and make it up later when she is able, because Allah does not burden a soul beyond its capacity. So she makes them up gradually, as her health allows.