You are a paying customer, there is no reason why you should suffer because of other inconsiderate passengers who break the rules that
everyone must agrees to at the time of ticket purchase.
Extreme body odor that cause a nuisance to other passengers for the entirety of the flight is
definitely ground for removal, unless it's a medical condition outside of anyone's control.
If you privately raise your concern to the crew before the plane takes off, they would discretely swing by to check it out. If they determine that the unbearable stench is due to bad hygiene, they would offload him according to the FAA rules instead of risking trouble once the plane is in the air.
If Someone On Your Flight Smells, Here’s What to Do
Can airlines kick someone off for body odor?
Actually, yes. “Most airlines have rules in their ‘contracts of carriage’ that allow an airline to remove a passenger if the stench is so bad that other travelers could potentially become sickened by the odor,” says David Reischer, attorney and CEO for
LegalAdvice.com. “Additionally, the FAA has operating procedures that allow for passenger removal if the safety and health of other travelers are affected by the bad odor from a passenger.”
As well, as private companies (at least in the U.S.), airlines generally “do not have an absolute duty to transport you under any circumstances, even if you have a ticket,” explains
Miguel A. Suro, a Miami-based attorney. “Indeed, there will be a contract of carriage between you and the airline, and it will probably contain broad language to the effect that the airline can eject you from the flight if you present any kind of problem.” Suro points to the current
American Airlines Conditions of Carriage, which specifies that a passenger must “be respectful that your odor isn’t offensive (unless it’s caused by a disability or illness).”
https://www.insidehook.com/article/health-and-fitness/if-someone-on-your-flight-smells-heres-what-to-do/