Yikes. Hate to hear that. I have a 135HO with about 110 hours. It has been awesome except for the odd time during hot/sunny/no breeze days when the 'hot air sensor' will sound after about 4 hours of trolling.
Hope one of the forum experts can give you some direction. However, a knock is something that I am sure they will tell you to get your friend to get that engine into a 'good' Evinrude Dealer to diagnose...
Can't really diagnose a sound on a computer screen and without examining the engine.
Unusual sounds should not be ignored. Your friend should have his motor checked out right away by his Evinrude dealer. The motor is probably covered by warranty if it is used as recreational.
"There is never just one thing wrong with a boat";
-- Travis McGee, main character in a book series by John D. McDonald
The factory recommends that a properly trained technician service your Johnson or Evinrude outboard motor. Should you elect to perform repairs yourself, use caution, common sense, and observe safety procedures in the vicinity of flammable liquids, around moving parts, near high-temperature components, and working with electrical or ignition systems.
The information offered here is only general in nature and should not be construed as complete factory approved procedures, techniques, or specifications. Always use the proper service manual for your motor, up-to-date service literature, the correct tools, and have an understanding of how to proceed with troubleshooting and repair methods. If you are unsure or uncomfortable with a procedure, a situation, or a technique, enlist the services of a factory trained technician.