Is it possible that the landlord had sufficient information to know that the parking lot was not sufficiently secure from car thefts, and that tenant's cars were at serious risk of being stolen, such that the landlord might be liable? I can't rule out the possibility, but I would hesitate to assume that there is a significant probability that the tenant could prove an inadequate security claim against the landlord.
The tenant's best bet is to make a claim with his or her own auto insurance, and to let their insurance company worry about whether or not the landlord could plausibly be held liable. If the tenant is insistent upon suing the landlord, the tenant can have the facts of the case reviewed in detail by a lawyer, who can assess the chance of prevailing in court.

