My missus found a good work around for clipless with an air boot.*Not a terribly helpful suggestion I know but if your friend has clipless pedals and shoes she could try only pedalling with her good leg on a much lower gear and just let the stricken foot either dangle or rest? As said above, not being able to flex the ankle will severely hamper any normal pedalling action. I'd personally not risk it but YMMV.
*I did this as a joke for her when she broke her ankle and she wanted to make her mates think it wouldn't stop her cycling.