You may have done this already but if it were me, I would get a statement from the client’s physician. The client is saying it is due to his previous injury and that may be the case, but you also need to make sure there is nothing else going on there as well. Only his physician can tell for sure and make the right diagnosis. If he jumps right in and performs exercises that may be contraindicated, he could further injure himself and complicate the problem more. Again, you may have done this already but you can never be too safe. Good luck!
to add to Seth’ comment in preparation for your meeting with your client:
I have often found that people who had to use crutches for any length of time (as I assume was the case for a broken foot) develop problems/imbalances in the hip stabilizers, and I would look at the relationship between the two. It may be the residual hip abductor weakness that perpetuates the ankle problem.
Here is a research link that talks about the subject, even though the circumstances are a little different http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1421486/.