Welcome! Great reward idea mate.. I hope we can all help as much as possible, if harmonica is proven to help stop smoking this site will really be in business 🙂
Re breathing, the train in lesson #1 is designed for just that, building up your lungs. Tale a look, but just remember to go slowly at first and aim to keep it going for about 20 – 30 seconds (you might need to look at a clock) .. once you have it going steady at the slow pace, over the course of a couple of weeks, start building up the time to go for around a minute or more.. it’s at that point that you start to speed it up, not before!
In the lesson, I speed up quickly to demonstrate, but that’s after 27 years of the train! So, yeah.. take it easy and do a little every day.. it helps for sure!
The other one is just to play the longest (comfortable) blow breath you can do through one hole, followed immediately by the longest (comfortable) draw breath you can play through the same hole.. and repeat..