
Do you know how much she weighs? As others have said a lot of Goldens are overweight if not grossly overweight. We're so used to seeing them this way a lot of people don't realise what a fit golden looks like.
I have a big Golden boy here, who's fighting weight is around 40 kg. Recently I noticed he looked slimmer. I started worrying about him, but eventually realised it was probably because we have a youngster here too (17 months) and our boy has been bombing round the fields much more playing with her. He's running more so burning off more energy. I've just been increasing his food slightly.
Try not to worry too much, I'm sure your girl is having a great time, although if you are seriously worried, I would probably take her to the vets for weighing, then re-weigh every few weeks to see if there is really a weight loss.
As others have said, Goldens should be able to work all day, so a dog which has free running regularly is building up muscle and improving fitness. At her age, it will do her more good than harm imo. A slim and fit Golden is much less likely to be afflicted with painful joints. Salmon oil is excellent as a supplement and all my dogs get it from time to time.
Good luck with your girl, it sounds like she's in safe hands during what must be a difficult time for you all.