Assuming the nut just needs pulling back against the frame again (similar to a pop rivet) have you tried placing a high tensile bolt in it with a nut wound half way out and several washers between it and the frame. Then tighten the nut down onto the washers whilst holding the bolt with another spanner or vice grips to stop it from spinning in the riv nut.
The nut should push the washer hard against the frame and then start pulling your riv nut back into the frame as you keep tightening it. Just make sure the diameter of the hole in the washer is large enough so the end of the riv nut you can see coming out of the frame in the photo passes through it. You may need several washers to make it thick enough so the riv nut doesn't push directly against the nut your tightening.. Hope this makes sense. Kind of hard to explain it but it should work.
Josh
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