Hopefully using the simple sentence "I have a medical condition, and my doctor has advised against using a hand wand near my chest" will be all he'll have to say for TSA to find an alternative method of clearing him. But carrying that card around might not be a bad alternative, just in case someone insists upon seeing some documentation.

Alternatively, he might want to make the screener aware that he has a medical condition and simply request a private screening to handle the matter. This should permit him to keep confidential medical information as confidential as possible, and keep his bags and papers within his control during the screening process as well. Just be sure to arrive to the airport a little earlier, as this approach will likely require additional time at the checkpoint.