Hope the thrush goes away, and if the horse is real lame it may be more than thrush.
Make sure the horse has enough feed / hay and water.
Start by having her brush, groom, clean hooves , lead the horse around. Maybe sit on him while someone is holding the lead, to figure out her balance . Then be lead at a walk , jog, then on a lunge line , working on balance. Then let her take the reins while being led, so she can work on work on light hands .. leg cues. Don't be bossy or pushy.
Most importantly be safe and have fun.