Sadly, most people don't know what their electric rates are, yet pay the bill every month. The answer is right there on the bill...
Where do you live? Electric prices vary widely from region to region. In my area, it's about $0.11/kWh.
A full charge is about 21kWh (useable), but you're only charging to 80%. I believe L1 charging is about 80% efficient. So 21kWh * 80% charge / 80% efficiency = 21kWh that you pull from the wall (if you were charging from turtle, which you're not, so it's less).
21kWh * $0.11/kWh = $2.31 worth of electricity
If your friend has double the rate, it will still be less than $5 to charge. Personally, I would suggest you offer to pay a little more than her rate, for the convenience and to build good will.