You simply cannot make the accusation that your colleague is somehow getting a beneficial working deal when you cannot know what the deal they have actually is ?
Whether or not they have an agreement (at least for now) of doing less hours for the same money is simply none of your business.
They may well be on a 'graduated return' to full time hours, or they may have agreed to only do part time hours and agreed to take a pay cut for this, rather than have to either go off long term sick or to leave permanently.
Whatever their 'deal' is, it's simply non of your business.
You need to ignore what your colleague is doing/not doing and concentrate on YOUR work. If you feel you are having to do more work, then address this with your boss, but do NOT say that it's because of your colleague !
Disability and any reasonable adjustments agreed with your employer should not impact other staff, but there may well be a conversation needed to ensure your work/hours/responsibilities are kept as they were, or a suitable financial reward is agreed if you are required to do more.
Trust me, no one ever wants to have their health issues 'aired out' at work with their bosses. It's NOT something that is done for an 'easy life' or for no reason other than laziness. It's embarrassing, soul destroying and takes months of meetings with bosses/unions , doctors/hospital etc which are all hugely stressful, and often impact existing health conditions even more.
So forget about your 'it's not fair' argument, as it's irrelevant and baseless.