Whilst C152 are all very similar to each other, there is a lot more variation between C150 models. A generic C150 POH downloaded from the web may be quite misleading.
The only safe approach is to use the one for that actual airframe. The aircraft owner should either allow you to access it to take notes and/or a copy, or give/sell you a copy. If they won't do one of these things, then they probably have other bad habits as well, and it's best never to go near the aeroplane again.
P