Anecdotally I cannot say it has gotten harder for me to lose weight. It is still as hard now at 39 as it was in my 20s.
As it turns out, the simple rule of "calories in, calories out" is true and possibly the easiest heuristic to follow to lose weight. She has hardly increased her metabolic activity (calories out), but primarily shifted her diet (calories in).
The mentality around weight management would do well to focus more on sustainable diet changes rather than on exercise (not that the latter isn't important, but that diet management seems to me more attainable and effective).
The only way I did it was to log all my food and keep to a calorie limit every day. I get people that ask me how I did it all the time and I tell them I ate less food and they all look disappointed like I was going to tell them some magic trick. I've managed to keep the weight off by really only eating one meal per day, in the evening.
Exercise is good and important in itself but it won't help you lose weight, outside of extreme levels of activity.
Recently I accepted a $500 challenge to get down to a certain weight by a certain date, so I started taking the calorie in- calorie out bit seriously. I won the challenge of losing 30+ pounds and in so doing became very suspicious of the low-carb mentality I had adopted 25 years ago or so when the problem really surfaced.
To get through cardio I need carbs, and suspect that I would increase my overall calorie intake to get them as I was keeping my carb percentage low. Stop the exercise and I could take the weight off.
So I'm definitely back to the opinion that calorie balance is the real key (short of some metabolic disease). I'm 61 years old.
When I switch to salads and cut sugar, I still lose weight quite quickly.