This is why I built my own ebike rather than build one. Bike specific batteries are great, but just like why we hate new 'standards' the real cost is when you need to replace stuff. For reference a quality 17Ah(36V) battery cost me $500.
Most ebike batteries are in fact just a bunch of 18650s stuck together.
did your friend get the battery replaced in the end?
i repack ebike batteries with good cells, spotwelded. sanyo ga are usually good cells. depending on location, there might be folks that could do it for reasonable costs..