owners of multiple dwellings (3 apartments or more) in New York City to provide, install, and maintain window guards when a child (or children) ten years old or younger lives there. Tenants with no children - or none living at home - may also request and receive window guards if they want them for any reason.
(this is applicable to condo/cooops also)
the owner is responsible and may charge up to $10 for labor reimbursement. But is is the buildibngs responsibility to comply with the law.

You can read more here:
http://www.dhcr.state.ny.us/ora/pubs/html/orafac25.htm

So is it the owner of the unit who pays for them or the co-op? I'm sorry but I'm not clear on what you're saying here. The link you provides mentions owners and tenants in rent stabilized apts. In the co-op, the shareholder is the owner, right?

If I were you I'd put those bars up and then work out the co-pay arrangements with the shareholder.

Braverman & Associates http://www.braverlaw.net/article_12.htm and

http://findarticles.com/p/articles/mi_m3601/is_n13_v40/ai_14636973

Richard Siegler, a partner with Stroock, Stroock & Lavan said the law is crystal clear....under city rules co-ops are treated as multi-family residential buildings, the board of directors acts as owner and is required to send out annual notices and supply window guards to anyone who requests them.

Mary Ann Rothman, executive director of the Council of New York Cooperatives, explained if a violation is placed on the building it would be joint responsibility. "A co-op has the full gamut of responsibilities that any other owner has," she said.

Simply put, the building (coop or condo) pays for the purchase and installation of child guards in all apartments. The owner or share hold of that apartment that is geting the guards is charged $10 per guard installed.
Pg

Thanks for the clarification.