Your brothers mate needs to clean his mowers better
Its true the body rots - its caused by the cut grass clinging to the bottom esp when used in the wet . Youd be better spraying salt water at it they would last longer
Ours got scraped after each use and pressure washed the underside once a week
We had a massive kawasaki powered etasia and a slightly smaller honda mower both were there at least 5 years and bodys showinf no signs of dying , the hayter harrier we had for fancy lawns was a bucket of crap and for ever falling apart - roller height adjust mech despite being used the least. My first year there we had a honda that was going into its 6th year and had had its body welded up a few times.
Ive got a suzuki 120cc engined flymo thing and a bush cutter for my own grass - never struggles - just a bit heavy to throw about like i used to ( we used to use it for cutting bankings on a rope before i inherited it )