Thank you all for your input. There are some good suggestions that we will try.

Please note that removing the battery to unlock the camera is not necessary. It just requires going to a wide zoom and shooting a picture at the locked f-stop. Then the camera resumes normal operation.

Sorry for the lack of clarity:
She is shooting in Speed Priority ("S"). The reference to "high speed" is continuous shooting vs single frame. As to the aperture "lock up", the lens is a fixed f2.8. The reference is to the zoom setting. She is usually shooting at 50 to 70 mm. She has to zoom to 24mm to get the camera to fire. Even at the 24mm setting the aperture remains locked on the f-stop until fired.
As to "locks up", yes the shutter will not trip but also the f-stop will not change, even if the lens is pointed toward a bright light source. So both aperture and shutter are locked. Note that the aperture was in range for the original shot. The lock up occurs after the picture is taken.

Thank you all for the replies. I discussed the problem with her and I also thought initially it was a failure to focus. However, if she directs the camera at a well-lit subject it remains locked up. I never heard of a focus failure that permanently locked the camera.

I’m new to this forum and have been impressed with the knowledge of the people posting. Hopefully you can help me.
I have been a Canon shooter for over 25 years, and now I have what I think is a Nikon problem and need help. My daughter volunteers at our local SPCA. She takes pictures of the cats and markets them on Facebook. She knows just enough about her camera to get good pictures. For the last two years she had been using a Canon T6s with a Tamron f2.8 24-70mm lens. She uses a rapid fire setting and takes up to 600 images a night just to get that “perfect look” for an animal that best expresses its personality. She usually shoots in low light conditions because she wants “normal looking” eyes, not slits (from flash). So in order to let her shoot at higher ISOs I purchased a Nikon D750 for her for Christmas. She also got the same Tamron lens for the Nikon.
Now for the problem. She shoots with a fixed speed (1/60) and at ISOs ranging from 1600 to 6400. She sets the shutter on high speed and is usually in the 50mm to 70mm lens range. Often during her session the aperture “locks up”. It can happen at 2.8, 8, or 11. In all cases the camera is frozen. The shutter is inoperative. She takes the lens back to a wider setting, shoots a picture at the frozen f-stop and then it frees up.
She never experienced this with her Canon camera shooting under similar conditions. I admit my lack of knowledge of Nikon cameras is probably the problem. Any help would be appreciated.