She wants to open her own window in the morning air, or shut it against the cold, without waiting for someone to come and do it. When the latch has stiffened, or the handle has worn smooth, the window becomes one more thing she has to ask for. Before you decide who fixes it, look closely at what kind of latch it is.
Local option: New Plan Care gives families in the St. Louis area in-home help so a parent can stay home: companionship, help with bathing and dressing, meals, and overnight care. Call (314) 405-0887.
What you can swap yourself
A surface latch, the kind that sits on the outside of the sash or frame with two or three visible screws, is often a job for one afternoon and a screwdriver. Take the old latch off first and look at the screw spacing, the direction the lever moves, and how far it sits from the edge of the window. Photograph it or bring it with you when you shop, because a replacement that matches the spacing will sit flat and seal properly.
Look for a lever she can push or turn with the flat of her hand rather than a small tab that needs pinching. If the new one is harder for her to work than the old one, it is the wrong part, even if it fits.
The general rule: if you can see every screw, take the hardware apart without forcing anything, and the window still closes and seals after you put it back, the job is yours to do.
When it becomes someone else's job
Stop and call for hands when any of these are true. The window is painted shut or has swollen in its frame, because forcing it can crack the glass or pull the sash out of line. The latch is set into the wood, so the fix would mean chiselling or cutting. The window has a stop or limiter that controls how far it opens, and you do not understand how it is set. Or there is an alarm sensor on the window, because changing the hardware can throw off the alarm's reading. For that last one, call the alarm company before you touch anything.
If this window is one of the ways out of her room, make sure that whoever does the work leaves her able to open it from the inside without a key or a tool. Test it yourself afterward, standing where she would stand.
What to check or ask before you buy a part
Ask whether the replacement matches the screw spacing you measured. Ask whether she can operate it one-handed from her usual chair or bed position. Ask whether the window stop still works after the swap. If someone else is doing the job, ask them to show you the new latch working before they leave, and to tell you in writing which screws they used and whether they touched the sash or frame.
Where to start locally
If the window is stuck or the job is bigger than a screwdriver, the local help page lists where to ask. For the reasoning behind this kind of guide, see how we write.