All travelers should know their rights and responsibilities relative to the seating policies of the airline. A person meeting the airline definition of person of size may opt to purchase one or more extra seats in advance. Alternately, if they board the aircraft and it comes to the flight attendants attention that they are a person of size, if there is an available seat they will be accommodated. If not, the policies say they will be de-boarded.
Likewise any person who is next to a person of size - you have the right to your space as well. It the adjacent person meets the airline definition of person of size and if it concerns you, then you may notify a flight attendant for a solution.
All of these situations can be handled like reasonable people. There is no need to involve emotion. There is no need for one passenger to discuss the issue with the adjacent passenger. The gate attendant and flight attendant are tasked with resolving person of size issues that arise according to airline policy.
All travelers are responsible for being an active participant in their experience. The best solution is to plan in advance and avoid extra cost or a drama at the gate or on the aircraft. If you choose to fly on an airline, you are bound by that airlines policies. The policies are very clear relative to person of size seating.
Likewise, if a person of size chooses to ignore the policy and take the chance of the adjacent seat(s) being empty, or of other adjacent seats being available - that is their prerogative, but they should be prepared for the consequences such as not being boarded, being de-boarded, and incurring additional cost. Lastly, if the person of size proceeds to encroach significantly on another travelers seating, it is fair to say that is inconsiderate.