How to remove this Shimano cassette (actually: freewheel)? A neighbour approached me to help him with removing the cassette from the wheel in the picture below. I am familiar with the modern Shimano lockring but would appreciate tips how to remove this one.
Thats a Shimano freewheel, not a cassette. You need a freewheel remover like Park Tool FR-1 to remove it. Note that this is not the same tool as the cassette tool (e.g. Park Tool FR-5 for Shimano-type cassettes). 

See this link on how to remove it with the freewheel remover (Recommended method), or this link on how to destructively remove it without the freewheel remover (not recommended; primarily for freewheels don't have a remover available).