The tendon can slip out of the hock joint. It will usually cause the leg to point away from the bird's body around a 45° angle but in severe cases It can be as much as 90° from where it should be pointing.
If you do a search in the Emergencies forum for slipped tendon, you will find posts with...
A guinea can adapt to only one leg but won't normally live long. A one legged guinea is far too susceptible to predators and can even have the flock turn against it.
Can't tell anything from the picture but a slipped tendon is not uncommon in young guineas.