Losing fat but gaining muscle with little to no exercising?

Hi everybody!

first post here. I am asking for advice for my girlfriend actually. She is 28 and has gained weight in the last 10 years or so to the point she is around 40lbs heavier than she should. She has never been much active but when a young girl used to bike and walk a lot and has gained a lot of muscle, compared to the overall weight, for her age. She has quite large legs but weak arms. To correct all this, she has been followed by a dietician in the last 2 years but with meager results.

At the beginning of this year she asked the dietician for a plan that would allow here to lose at least half the amount she has before our summer vacation in September. The dietician was positive this was perfectly doable and prepared a diet of about 1750kcal for a TDEE of 1680kcal. She gave her some light exercising to do 3 days a week.

The issue is, due to covid and other health issues, neither me nor her have been exercising regularly in the past 3-4 months. Some weeks we do as planned, other weeks we completely skip. We don’t go to the gym but train at home. My gf has stomach issues that we need to investigate about that cause her to switch some dishes every other day because otherwise the acidity is unbearable. She is going to get tested this month about this.

So, despite not following religiously the diet nor exercising much, she has lost about 3.5lbs of fat while gaining 4.5lbs of muscle since February. She is at the same point as she was, more or less, months ago but with less fat and more muscle.

The dietician doesn’t understand how she could have gained muscle while eating barely the TDEE and being incostant with exercising. She suspects myostatin-induced muscle hypertrophy. We don’t know yet for certain though.

My question is: if it were true, what could my gf do to steadily lose fat? She doesn’t care about muscles nor want to join a gym and is actually scared more regular exercising would only make her bulk more rather than slimming down. Anybody has any suggestion or a similar case?

Thank you in advance!

Losing fat but gaining muscle with little to no exercising?