Like the old saying goes, "If you don't know how to tie a knot, tie a lot!"
But gee whiz... Okay all you guys out there who don't know how to tie two ropes (or sheets) together, study this: The Double Fisherman's Knot. Absolutely the safest knot for the purpose, imo. Your life may someday depend on it.
You can easily make it a triple or quadruple fisherman's knot if it'll make you feel better, and a lot of climbers do when using rope. Don't trust your life to a simple sheet bend or a square knot or, god forbid, a granny knot! Also, if you're tying sheets together to shimmy down, and you have lots of sheets on hand, twist or braid two or three of them together for each section of your "rope."
Hope this poor guy isn't paralyzed for life.
They were probably working in the dark too, unless she had a flashlight in her purse.
The sheet bend (also known as becket bend, weaver's knot and weaver's hitch) is a type of knot, related in structure to the bowline. It is very fast to tie and is useful when joining two ropes of different diameters or a rope to a sheet corner. Along with the bowline and the clove hitch, the sheet bend is often considered one of the most essential knots, being shown as knot №1 in the Ashley Book of Knots.[1] It is a more secure replacement for the reef knot (square knot), especially in its doubled variety.[2]If the two ropes to be tied are of very different diameters or rigidity, the double sheet bend holds more securely. The only difference is that the smaller rope wraps the larger rope twice. As with the standard sheet bend, the two free ends come out of the same side of the knot.
Wikipedia