She doesn't sound weak-minded or physically lazy to me--it sounds to me like your problem is that she doesn't care about about what YOU care about. Did you notice that YOU are the one who is always designing her diets and trying to get her to follow them, that YOU always telling her to do cardio and monitor her heart rate, that YOU always telling her not to eat chocolate, etc?
You said she came from a sport's background, which tells me she does NOT have an aversion to physical movement...because physically lazy people avoid ALL physical exertion, especially sports. You also said she has no problem walking for several miles at a time when she's going somewhere she wants to go.
I don't see a lazy girl here. I see someone who just doesn't care about the physical activities you care about...someone who doesn't have the same physical goals that you do...and someone who isn't interested in mimicking your methods to attain health and fitness.
You say she doesn't want to follow a bodybuilding diet like the other 99.9% of people on this earth? Well that's not surprising because it FUCKING SUCKS! You say she doesn't want to live a bodybuilding lifestyle like the other 99.9% of people on this Earth? Well that's not surprising because it FUCKING SUCKS!
Very FEW people actually prefer the bodybuilding lifestyle compared to normal living, including bodybuilders...because it just isn't fun to cause physical pain to oneself every day, eat things you'd rather not, and run in place for 30 minutes while staring at the wall. If most bodybuilders, even pros, could build an equally good physique eating the foods they love, never doing cardio, and only training hard when they felt like it, the VAST majority would choose that lifestyle instead of their current one.
Why do think almost NO retired bodybuilders continue doing what they did when they were competing, drugs aside? Because they no longer want to eat or train like bodybuilders. Nearly all of them end up saying to themselves "fuck that", I don't wanna live this life anymore. They are no longer concerned with having a world-class physique, so they stop living the lifestyle. If they loved the lifestyle more than normal living, they would continue living it post-retirement, but they don't...because they don't! Even Dorian Yates, which was one of, if not the most dedicated bodybuilder of all-time, stopped living the lifestyle post-retirement. in fact, he went in the complete opposite direction and started getting high, drinking alcohol, and eating 2-3X a day when he felt like it.
Anyway, I am rambling, but the point is that by the way you made things sounds, it doesn't seem like she is a lazy person. Rather, the issue seems to be that you want her to be a bodybuilder (at heart) like you and she doesn't want to be. Anything bodybuilding related she does participate in is probably only being done to please you. You think she doesn't have the willpower to follow a diet for a single day? Not at all. What's going on here is that she doesn't WANT to follow your diets, so she isn't, but instead of coming right out and telling you that, she is being passive about it and expressing her disapproval by never actually doing it. This way she doesn't have to deal with confrontation and go through head-on war with you over why she doesn't want to live the bodybuilding lifestyle. I bet if she has a physical outlet she enjoyed--maybe with some lady friends--she would have no problem getting involved.
If you can't accept the fact that she is probably NEVER going to consistently follow a bodybuilding diet, train with a die-hard mentality, and get excited to do cardio...then you are probably with the wrong girl. Remember, it is YOU that are the oddity in this relationship--not her. That doesn't make you wrong, but it doesn't make her wrong either.
On another note, here's a little tip. NEVER design diet/training programs for friends/family members for free---because free diets have no value. If someone doesn't have to invest anything to get something, its perceived value is nothing. On the other hand, anyone who PAYS for diets/training programs is going to place a whole lot more value on it because it cost them something. If a friend/family member isn't willing to pay even $100 for your expertise, which could change their lives, then they definitely aren't going to follow a free diet. Stop wasting your time. The people who get free diet/training plans are ALWAYS the first people to flake out.