Ouch.I had 2 stress fractures last fall,and sat out until january(total of 4 months).Both on the l-3
What worked for me.
-Start out by working with a stationary bike.Legs are probably very stiff
-Stretch daily,in particular your hamstrings and calves
-do light weight work,and build back up to where you where.I assume he was in a backbrace.Wear it for the first few weeks of workouts.
-swimming is great
-lots of ab work
-Medicine ball rotational movement,when done moderately,can help gain back hip movement/flexibility
-Stretch Bands to help build back arm strength w/o pain worked for me
Also,one thing I have found to work sounds funny,but I understand it helps.While lifting,it helps to squeeze your butt cheeks together,as it stabalizes your spine.
Not a doctor,but when I was recovering from this injury,I did all of the above.Tell him to be patient,and dont rush.He will only re-injur himself.