Concerns re: my granddaughters weight

I raised my granddaughter as if she is my own as her mum is, unfortunately, not present in her life. I tried to shield her from her mum's escapades as best I could , tried to compensate for it as well. When she was little, I admit, I OVERcompensated from time to time. By treating her to sweets, pastries, anything she could possibly want. So her weight has always been a concern. Starting from when she was about 12 years old my DH and I tried to turn things around and made effort to keep her at a healthy weight. We made exercise something we could all do together rather than something that would make her feel "different", all three of us ate healthy meals.

She was receptive to it for a time, however, she stayed a heavy child, grew into a heavy teen, and then eventually, an overweight young adult. Now she is 24, and though I wouldn't describe her as morbidly obese she is chubby. And getting chubbier and chubbier. Our concern is that the number on the scale isn't stable; as a matter of fact, it is only going up. We have been very careful about how we approach the subject since she is a sensitive young woman and has sadly dealt with her share of judgement and prejudice. (Really, the last thing we want to do is shame her.)

BUT, we also can't stand by and watch her continue to gain weight when it is a fact that carrying excess weight for so many years has consequences. At a point we considered whether her boyfriend might be someone who could help since he himself is very fit. He eats healthy, spends a lot of time in the gym and plays a number of sports. Unfortunately, from what we can see, his habits will not be rubbing off on her anytime soon. He seems to be enabling her. Takes her out to eat to all of her favorite food spots, buys her pastries, cookies, ice cream.

I should say that she is happy with him. He treats her well, is very respectful towards her AND us... DH and I are very pleased to see her in a loving relationship. This is her very first serious relationship, and after everything she experienced growing up, it means a lot to us to see someone making her this happy. We just wish he was encouraging her to be healthy, not to eat herself to an early grave. An ugly part of me says that if he truly loved her as we do, that is what he would be doing as opposed to going along with her habits. Still, I don't mean to interfere. She loves the boy.

A very long story short, DH and I have tried just about everything we can think of. What we hear from her when we talk about it, is that she is happy with herself, and her boyfriend is happy with her too, so why should she change? And to be sure, I understand. I'm aware she is an adult now, and ultimately her body and her choices are her own. But it is, also, incredibly stressful to see her make a lifestyle choice that I know is going to lead her down a very sad road.

Am I being unreasonable? If not, what can I do, besides express my concern, which is never going to get anywhere? How can I protect her without completely shutting her out?