The sad reality is, we don’t even need to call the RSPCA - it is almost inevitable he will begin to have health problems and she will be forced to take her to the Vet.
If I’ve learnt one thing about Vets, they have a good “bedside” manner but will be the first to call you out in favour of the animal’s welfare!
Our dog was 0.1kg over her recommended weight a couple of years back and were given a whole list of recommendations! It was just after Christmas, so the weight fell off naturally when we stopped the extra treats (much to her disappointment, but you’ve just got to ignore those spaniel eyes!) and went on longer walks. It was the tiniest “blip” but we took responsibility for it and her welfare!
I really hope the vet calls her out when she has to take him next, and I can foresee that being sooner than later