The short answer is yes, it can depreciate. However, during the past housing crisis where housing depreciated immensely throughout the country and foreclosures happened all over, guess what? New York City and its suburbs NEVER suffered that much. This is because co-ops require specific down payments and were forced to turn away buyers who couldn't afford it or who had financial statements that didn't support long term income to pay for their mortgage and maintenance fees. During the housing bubble (pre-2007), lenders were giving away loans to everybody whether they qualified or not. Co-op Boards were only accepting buyers who actually did qualify, dismissing everyone else even though they got pre-approved.
This prevented units in co-op buildings from the housing recession and declining in value. Reality is they declined a bit, but not as much as everywhere else. Currently, co-op buildings are rising in value again because inventory is tight.
Tell your client that purchasing a co-op unit is an excellent investment.