1. major swelling
2. compartment syndrome ......pressure within the compartment gets too high for example, from too much swelling or bleeding after surgery or injury, the lack of oxygen to the tissue can damage blood vessels and nerve and muscle cells.
3.blood clots
She is wise not to go....she needs to lay low with her ankle elevated above the heart to prevent swelling and other problems.
The Airlines of the past would have accomadated her medical needs.
But now adays she would have to buy the pillow and the extra seat space to elevate her foot.
Not to mention if she used a WC she might get left on the tarmac.