It’s really impossible to say unless we know how much weight has been gained and whether she was too thin before. But the idea that your body is putting on weight as an adaptive measure to restriction is not unwarranted but your body can’t create fat out of thin air or starving people in South America would be rejoicing. The weight is from food and at the end does it really matter whether it’s an adaption or not? The end solution is still learning to have a healthy relationship with food, learning that it’s a fuel you can have too much of and eating less calories to negate the “adaptation”. Unless you’re okay with being overweight in that case I think that’s still a conversation you’d have to have with your partner and then they would have to decide if that’s someone they want to be with.