I’m a custom wheelchair provider. What you are asking for doesn’t exist. There are some nice half lap trays which do flip away when not being used you can get on Amazon for between $100-200. When you flip the tray away it will rest against the rear wheel. This means the tray will rub against the rear tire when flipped to the side. They flip to the side for transfers. Not for the chair to roll with the tray in that position. It would also damage the tray to be banged into doorways. I’ll include a link to a tray on Amazon. Look carefully at the different styles. Padded are usually provided to support a non-functional arm. Plastic is for functional for writing, reading, or supporting plates/cups.

SHE SHOULD NEVER USE A TRAY FOR SUPPORT WHILE STANDING. IT WILL BREAK AND CAUSE A FALL

Please send a message if you have any questions I can help with

https://www.amazon.com/Rehabilitation-Advantage-Polycarbonate-Wheelchair-Rubber/dp/B00GC5BMSQ/ref=sr_1_8_a_it?ie=UTF8&qid=1543293960&sr=8-8&keywords=wheelchair+tray+half

I’d suggest sitting down with her and show the options and all the considerations so she can choose the best fit for what she needs

Last thing, the arms on the chair are desk length (about 9” long). There is a full length arm available (14” long) which will support a tray like on the Amazon link better. Her wheelchair provider may be willing to switch the assembly at no cost if you bring it to them