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Well... tell her to bring the beer and cookies, but DON"T SIT ON THE CAR (problem solved).
I don't care if it was a woman, man, my own children, parents, or my most trustworthy friend/family ever I would never trust the OP question about placing jackstand at diagonal corners.. You can pick up a nice set of jackstands at a pawn shop like new for under $20... so skip the cookies, and beer for one oil change, or use conventional to run out the normal OCI (of course if your engine specs conventional) and forget any frugal jackstand placement.
Now if your wanting to spread them further apart in the front while working under the car, then you can support more weight working with a rigid structure, and having a rigid structure for support with the jackstands spread further apart BUT you must use the points intended for placement of jacking/lifting or when the car falls it will just crumble the parts supported on the car itself, or knock the jackstand out of the way.
I get nervous about jackstands myself, and I hate them. If changing the oil I will choose the rhino ramps at walmart every time, which is in my opinion the best way to change oil in a low to the ground vehicle. I also understand the ramps can not be used while doing many repairs on a vehicle.