Could well have been deep vein thrombosis from the flight from Ukraine.
DVTs are simply the existence of a clot in a major vessel in the lower half of the body. In and of themselves, they aren’t immediately life threatening. It’s when they travel is when they become life threatening. If that is a case here, then the question would be where the clot ended up to cause the need for medical intervention.
If they were perceiving him as having a “sudden illness”, my guess would be a Pulmonary Embolism (aka, blood clot(s) in the lungs). Many of its symptoms mimic common respiratory illnesses like shortness of breath, coughing (with or without blood), and the like. With him having just taken those multi-hour flights to and from Ukraine, many warning signs of DVTs (For me, the only warning I had when mine developed was something like a nagging cramp in my calf muscle*) might have been chalked up to the extended travel. Respiratory issue might have been downplayed as catching something from Ukraine and they’d see how he did over the weekend…until the clot occluded something important.
*Yes, I have had a couple DVTs in my lower leg. I AM on long term blood thinner for the issue. Again, the only indications I had a DVT were low end, persistent, nagging cramp like feelings in my foot arch and calf muscle. They were caught soon enough that, with treatment, I have not had any issues aside from long term vascular damage causing one of my lower legs to be more prone to edema (fluid retention/swelling)