Center Controller reported due to passenger illness; pilot descended below minimum altitude in the area resulting in a CFTT event. Pilot diverted and landed safely.

Narrative

Aircraft X was on an IFR flight plan to ZZZ at 100. The pilot was informed that a climb to 110 would be necessary for high terrain west of ZZZ VOR. Pilot stated that they will be able to climb to 110 when needed. As the aircraft was approaching terrain as well as the next sector (sector 17); I issued a climb to 110. Pilot readback and began to 110. Shortly after the pilot had leveled off; they alerted me that they would need to either head back to airport of origin or antoher airport. The pilot then started to descend to 100 without a clearance. I started issuing the terrain to the pilot and that the minimum IFR altitude in his area is 095. The pilot was clearly flustered and continued to descend. The pilot was the leaving 100 and started to descend to 095. I asked the pilot of they were [requesting priority handling] which he replied that he was. I gave him 2 options for nearest airports; which were ZZZ1 and ZZZ2. He was indecisive at first so I asked if he would like ZZZ1 since it was a few miles closer. He let me know that is where he wanted to divert to and I cleared him. I asked him the nature of the situation and he let me know that he had a passenger onboard that was experiencing altitude sickness. The aircraft continued to descend in which I specified once again that the minimum IFR altitude in his area was 095. They ended up stopping 003 below the MIA at 092. I asked the pilot if he had the terrain insight as well as the higher terrain at his 1 oclock insight (120) and he replied yes he has all of he terrain in sight. I issued an altitude alert and once again stated that the minimum IFR altitude in the area was 095. I coordinated with ZZZ Approach about the nature of the situation and they had radar contact on the aircraft. I gave the pilot a frequency change to ZZZ Approach so they could better handle the aircraft and work them into ZZZ1. The aircraft landed safely at ZZZ1 a few minutes later.Recommendation: Nothing could have really prevented an event like this. Passenger got sick and the pilot had to take action. I understand why the pilot was trying to descend; which is why i continued to reiterate the MIA in his area.