Is it legal to have POA over a client you work with?

I agree with freqflyer. The only thing you want to check is the laws on paying the POA in your state to make sure it would not be considered a conflict to be both employee and POA. Some states are a bit skittish about non-attorney POAs who pay themselves. Other than that, there shouldn't be a problem.

A person can have anyone be their Power of Attorney as long as the person doing the picking is of clear mind. So if your client decided to pick you to represent her, when she can no longer make decisions, that is her right.