As a general rule, if you are not a medical professional and you have a client who has a pre-existing injury, you should always obtain clearance from their phyician before starting. Otherwise, if you attempt to "treat" the problem, you're stepping outside your scope of practice and can open yourself up to issues with liability.
One helpful piece of advice would also be to have your clients fill out a pre-screening form, alerting you of any prior injuries or issues they may have had. If you had your client fill this out, you may have seen that he had issues with possible neurological weakness in the upper extremities which would have been a red flag before getting started, especially with exercises that require upper body control as the TRX does.
The TRX is an excellent tool, but it may not always be the appropriate place for everyone to start.
Ada Wells, MPT
Physical Therapist