iDrive35I sounds like Apple product and it is not listed as a trim for this model on TireRack.
Is it sDrive or xDrive?
Then there are 4 models:
Base
Luxury Line
M Sport
xLine
Each may have a little different wheel requirements.
When "BMW tech" stated that square setup may be run did he mean that car accepts such setup but one needs to replace wheels or it can be done with current wheels? The rears may be 1-2 inches wider than fronts.
The cheapest set on TR in original staggered sizes is 870 USD. The most expensive is 2000 USD.
None are cheap.
I think the biggest question is: all out performance (summer performance tires), longer treadwear (that may be hard, depends more on driver and roads - all season, choice between grand touring and performance or comfort - grand touring.
After picking what kind of performance is wanted/expected then start looking for tires. If RFTs are a must that will limit what is available.
Good luck to family member,
Krzyś
PS And why is it "snide remark" to mention that owning a car is more than just buying it? I refused to buy 19" optional wheels and looked for basic 18" (sounds strange already) just to save on expense while loosing comfort.
Staggered 20" price range on TR as mentioned is 870-2000, Squared 19" is 500-1670.
PS2 Wheels
According to
https://www.wheel-size.com/size/bmw/x5/2015/
The car has:
F 10Jx20 ET40
R 11Jx20 ET37
And 275/40R20 tires have rim range 9-11 so you can go square with front sized tires.