Id imagine your biggest issue up there is moisture BLR...
Youre right, in a thermal break is best, and necessary to properly insulate...and if the temp hits dew point inside that room (that will be different to what is forecast by the BOM as it is its own climate in the shed), you have liquid water on the inside of the walls...add a breathing human & you have stale moist air...that equals problems..so you ideally need to vent this space to get rid of the moisture--all a little excessive for a garden shed...and.just foil will do nothing--or SFA anyway--it wont stop the immense radiant heat from .42 sheet steel in the sun, & by itself is incapable of retaining any heat...
Then when you seal the room, you have rising moisture from the ground to deal with (unless there is a membrane)...this creates issues with mould etc...
But then i dont really know your climate...
Ventilation is your friend...But if she is insistent, just make sure you dont break any rules & take the money i guess...
Id make it known you are not aware its going to be used as a habitable space--in other words, as far as you're concerned she just wants to make life a little more comfortable for her mower, hammer & screwdriver set...
Just remember It is a class 10 building, & for it to
legally be a class 1 habitable space, you need more than a roll of foil sarking...
Im a believer that the insulation ability of foil wrap is
entirely dependant on the system of insulation used, so to go by the stated values of the product alone is simply just denial...or ignorance...or both...
The way energy certificates are issued in our country is nothing more than a fuking joke & just a cover for greeny pushing Uni grads & bureaucrats who are insistant on being right--these "energy efficiency" requirements & listed ratings are in actual fact mostly worthless the way we construct normal housing here..
.
The man who knows everything, doesnt really know everything...he's just stopped learning...