Mom is staying at my house for one night in a queen size regular bed. She’s bedridden and I need to change her diaper. How do I roll her with no grab bar?

Where I worked we had a lending closet. Redcross has these too as do other types of organizations like the Lions club or Ruritans. Check with ur firehouse. Go on Neighborhood.com to see if anyone near you can loan u one. You may be able to borrow a grab bar.

You can find a simple bed rail that is secured under the mattress on Amazon. Its fairly inexpensive and would be a great help.

Queen sized bed is fairly big. If she lays right in the middle of the bed, it will be very hard for you to reach her. So have her sleep on the side of the bed that is easiest for you to reach. When you roll her on one side or the other, put a firm round body pillow right behind her back to keep her stay on her side.

Check YouTube, there are many videos on how to change diapers for bedridden patients in regular beds.

Another option is not having her stay the night if you don’t think you can handle this task and others that she might need.

I'm going to assume you've changed her before and understand the basics but if not you can look up YouTube videos that show how this is done.
Pull the bed away from the wall if necessary so you have access to both sides.
If she uses the rail to help you perhaps you could buy or borrow a bed assist rail... the most basic ones are fairly inexpensive.
If you place a draw sheet or large waterproof mattress pad (I recommend having one in any case) below her you can use that to turn her on her side.