Since the joint has completely failed, the joint needs to be completely and carefully disassembled. This should be possible since the surfaces on each part is not aligned anymore, although it looks like a portion still may be glued tight. From what it looks like, it still should come apart.
Just as mentioned in another answer, all remnants of glue needs to be meticulously removed being careful to remove the glue only and NO wood. This will particularly difficult at the edges. I use an incredibly sharp paint scraper for this, for the large open flats but the surfaces inside the dado where the working space is only about 1/4" to 3/8" wide, a sharp broad wood chisel may be an option to use in this tight space. A custom made scraper would be the best, and a file or rasp kept absolutely flat would be my last choice.
After all joints are cleaned of old glue, (by the way, it is needed to be cleaned, because the old glue is what failed, if it stays, it will fail again) assemble the part dry, t be certain the mounting holes are in the same place as they once were before it broke. Do this on a piece of plywood on sawhorses for example or at least elevated above another surface so it will help in a future step I will describe later.
Once laid out on the plywood, together as if it is not broken locate blocks of wood and 3 holes as I have marked up your picture here.
Take the time to set all the wedges and tweak their placement to get the joint drawn tight. When it is to your satisfaction, and the mounting points match precisely what the other half needs, add more blocks in key areas if you like to pinpoint easier to get critical alignment. Two places I would use are the 2 ends, the bottom of the leg and the part that attaches to the back.
All that done you are ready to glue. Mark all locations where the wedges were at, how deep they were set. Use a brush, I use a small brush like the plumbers use for applying flux to pipe before soldering. It has stiff bristles and can stand getting jammed in tight spaces. Wet ALL meeting surfaces and set the parts back together. Move quickly the yellow carpenters glue has a pretty fast grab time, and that time will pass by pretty quickly. This is why you did the dry run earlier. You will not have time to tweak while you are clamping it up. Murphy's Law will get in the way enough already, if you hadn't noticed while setting it up without glue, it changes when you add glue, it will not be the same, be ready to tweak one more time, mainly the floating tenon that holds the joint. You may even want a wedge block put there after you are done to drive it in flush. I just realized I typed all this with out mentioning how to clean the joint from all the glue mess that will occur. The hole at the elbow needs to be elongated to expose the underside of the joint, so a damp, not wet sponge can be used to clean the joint of all the drippage that will happen. Squeeze the sponge out as much as possible, the excess water will go in the joint and wash out the glue. This will happen real easy, don't underestimate it.