Seating design standards cater for statistical populations.
Without digging into specific research for your machine, the seat probably is designed and tested to accommodate a 215 lb body for the design loads. On top of this are various structural factors inherent in manufacturing and design generally.
On the surface of it that suggests that the significantly heavier pilot/passenger represents a gross overload of the seat. However, this occurs every day of the week in all sorts of aircraft, particularly airliners .. I can recall a trip back from Auckland when I was working over there some years ago ... seated between two ample Island folk.
In reality, what it means is that the crash load restraint capability is reduced in proportion to the overload .. ie the seat will break at a lower G-loading than it would for the design occupant weight. Whether you might want that situation to exist is not for me to dictate but introduces a duty of care consideration for commercial operations, I suspect ?
In respect of POH limits, the only restrictions I have ever seen (specifically for a seat) relate to seats loaded with (for example) freight where the maximum amount permitted is limited by the seat design load.
So far as the loading system is concerned, unless there is a specific load limitation imposed in the POH then the loading system can be developed to accommodate an obese person .. noting that it may be necessary to develop a loading system to suit the additional weight but that is only a simple load control matter which your local weight control officer can attend to.
The loading system, of course, will need to address keeping the loaded aircraft within the approved CG envelope throughout flight.
Floor loading is unlikely to be relevant to this Type although I have to caveat that with a note that I don't have the POH to hand. The 206, being a trash hauler, on the other hand .. will certainly have floor loading limits.
The 120kg "limit" suggested for the seat in the OP sounds like a bit of waffly nonsense to me and, I note, also djpil .. who is a very experienced aircraft designer chappie.
Following up on the later comments -
Got some more info today off the CFI when i questioned him his reply was " the weight limit is set because if you crash the aircraft the seat could buckle under the weight SO I WOULDNT BE COVERED BY INSURANCE" so its not a personal thing its a money thing.
... that's just arrant/ignorant nonsense - the seat will suffer damage once the combination of occupant weight and G-load (considering fudge factors involved) exceeds the seat's design capability. I think that one would have to acknowledge that the seat is "compromised" once the occupant weight exceeds the design figure (probably 215 lb).
I think the main problem relates not to the seat but to potential medical problems associated with the degree of obesity ? (I can see djpil saying to me that I should lose a stone or three ... so it affects many/most of us ...)