Possibly discrimination, possibly not. Not really enough evidence from what you’ve said to really say.
How do you know about the other student, and their reasons and appeal outcome? If they told you, maybe they’re missing out info that might be pertinent to the unis decision or obv if based on gossip, then it’s just gossip.
Also, to the other point, if you wouldn’t want to do the exam if you are successful with your counter appeal then you need to decide do you really want it? After all, you can’t get the full without doing the exam.
Its illegal to discriminate on the base of disability, but maybe it’s a case of your argument was not strong enough or clear enough on how these impact on your previous retakes.
Possibly too, if you have ongoing health issues than you’ve informed the uni about already, and the appropriate accommodations/adjustments were put in place as well as you are aware of their impact and the necessary prep you need for it. From their POV it might not be clear what was different for this exam compared to the others, and what was other wise different for this exam and it’s retake and what benefit to doing it again will bring, so they might not be convinced that you need another retake. Essentially, how many are they going to let you take..until you get the exact result you want?
The other student’s issue sounds it was probably something unexpected and a mitigating circumstance that impacted their retake in this specific instance that warrants another retake.
You can appeal, but worth thinking what to do if they reject it again. It might be worth checking how to escalate this if needed, but worth asking yourself as well is it worth the hassle. Try talking to someone too for more details on the decision.