So there's some misunderstanding within your post, which is totally fine because "healthy eating" is a weird, nebulus concept to being with. To begin with, butter and bacon fat are not inherently unhealthy. While they can assist in leading to health issues (mostly due to their caloric density but some people have higher sensitivities to saturated fats as well), eating a couple strips of bacon a day is not going to put your dad in a premature grave. In fact, he seems to be moving in the right direction, right? Health markers are moving positively - BP, resting HR, lipid profiles, less pills, blood sugar leveling off, etc. These are signs that his health is improving. So let's get that out of the way, simply choosing to use butter isn't going to kill him. We can get into this for ages for what's right for whom, but look at the end results more than the inputs to determine if it's working (fruits, veggies, meats, some carbs. All totally cool.)
Now for his rapid weight loss. How tall is he? If he's 5'5, then this initial flush of fat dropping is Totally Fine. If he's 6'5 then it's probably too fast. The concern, more than the hard cut, is the fact that it's unsustainable. He probably isn't on a 1500 calorie diet, but it's probably pretty low. Is he going to be hungry losing weight? Hell yes. Everyone who is reducing their body weight under their body's current set point is going to be hungry. Alternatively, anyone trying to gain weight appreciably above their current set point is going to feel full all the time. It's Totally Fine. What isn't fine is that if the weight loss is accompanied by moodiness, irritability, sleeplessness, significant lethargy, short term memory issues, etc. There's a wide range between healthy weight loss and starvation-level weight loss. 1500 calories is high enough to sustain the body of a relatively inactive person for a long long time.
My best tip is to support the current weight loss, and watch for the bounce behavior. His new body will not sustain if his old habits come back. This is the most aggravating part of a dieter. They think their old diet was not the issue, that their weight was. So they go from normal diet -> weight loss diet -> original normal diet and end up exactly where they used to be. There needs to be a New Normal Diet, which is going to be closer to the weight loss diet than the old normal diet. Less butter than before, more than current. Less drinking that before, more than none. Cut out the shitty carbs if he's not active.
Also weight lift. Because it's always a good answer.