I don’t know the model number of the motor, but it looks like Yamaha made a 2.5hp motor in 2015. I was reading in the owners manual about internal fuel filters. If the the fuel filter has some debris in it, it could affect fuel delivery. At higher speeds it may not be noticed but at lower speeds/idle fuel delivery maybe impaired. Sorry, I do not have experience with the Year, Make and Hp of the motor.
It just seems like the issue is fuel delivery.
Here is a link to what I believe is a owners manual for your neighbor’s motor. Page 83 reviewed filters and maintenance.
https://outboards.yamaha-owners-manuals.com/om/LIT-18626-10-21.pdf
Cheers,
Cajun
