No need for any risks. Simply have Apple develop the allegedly needed software in house and use it on the one phone. Simple. Then give the data to the FBI.
Isn't that begging for chain of custody issues? Also, once the phone is unlocked, the FBI could reverse-engineer the firmware.
Either way, this isn't really about this one phone. It's about the precedent. The idea that the FBI could force a company to make a broadly applicable security hack for its own devices is pretty chilling.