Ask the owners what they would like done with her. I would definitely start with ground work, even if she is good under saddle, ground work helps to establish the tone for the whole interaction. Start by getting her to yield all parts of her body off the ground, get her really focusing on you. Lunging with lots of direction changes and transitions.
Once you have a solid foundation on the ground start with the basics under saddle, if you don't know her history there could be large gaps in her training. Just because she was used as a lesson horse does not mean she is well trained. Start with walking, get her really listening to your leg and seat and responding well, then throw in some trotting. Start with minimal amounts of trotting spread out with lots of walking. Transitions are great for building muscle and if she is very out of shape then long stretches of trotting will be too much. Take it slow, build up to where she is trotting well, downward transitions with minimal rein, engaged and bending nicely, then throw in some canter. Same thing start with lots of trot/walk with minimal canter built in.