You can rent small units at storage facilities in Harrison, NJ for ~35/month. 10 minutes from the holland tunnel (she'll only have to go there twice a year).
Queens would be closer from the UES but more expensive. Long Island City is the neighborhood of Queens closest to her (directly across the Q'boro bridge). Google/yellow pages to call them
Current: E60 535i M-sport
Prev: E90 330i, E93 335i, W212 E350 Bluetec