I never learnt how to deal with a toxing diver in any of my TDI classes (Deco procedures and Normoxic trimix)
The internet has its uses
"Alert Diver" by DAN recently had an article about a wreck diver who switched to the wrong deco mix, and after about 10 mins he tox-ed, and he was rescued by a nearby buddy in the group, and fortunately he survived.
The main technical issue is to make sure you switch to the right mix, with lean to left, rich to right. For divers who wear all their deco tanks on the same side, switching to the wrong mix is a greater risk, and so these divers need to check each other before they switch, etc.
Secondarily, if your dive plan approches the Ox Tox limit (set arbitrarily as the equivalent of ppO2 of 1.6 for 45 mins, adjusted for depth and ppO2), then you need to take a back-gas break of 5 mins.
So, step 1, avoid getting into ox-tox trouble.
Step 2, to rescue an ox-toxed seizuring diver, put his reg back into his mouth, do your own deco, and bring him back up to the surface with you.
I am actually surprised that the guy in the article did not drown.