Hi TimPZ, and welcome to the G!
I'm assuming that the glue has failed and you need to rejoin the frame.
If the joint is held together with wire brads, a traditional framing vice can be used to help pull the rails apart. One leg is secured in the vice, but the other is on the outside of the adjacent vice jaw rather than inside it. The vice jaw is then moved outward in successive stages and used as a fulcrum to lever the rails apart. Depending on how it is nailed will determine your chance of success.
V-nails are a different beast, but if you have a joint where the v-nails have not been stacked, they can be pulled out at the foot of the frame.
There are a couple different methods. One involves excavating some of the wood from around the exposed end of the v-nail until enough is exposed that you can get a purchase on it with a pair of flush-cut wire cutters. The other is to drill a hole (3mm or so) into the foot at a shallow angle that also makes a hole in the v-nail. A small diameter steel bar is inserted and used to leverage the v-nail out enough so it can be removed with pliers.
No guarantees with either method, and Ylva's suggestion of frame replacement is often the fall-back solution.
Another thought is to simply knock the rails apart and resurface the joint, making the frame slightly smaller, if that is an option for the customer.