I’ve been on sertraline for decades, on various doses and have come off completely at times (although always had to restart although I’m on my lowest dose now.)
I would suggest doing it very very gradually and actually since they are on 75mg which is not the lowest prescribed dose, I would look to reduce to 50mg to start with and then stay on that for a while before reducing further.
There are 25mg tablets which are helpful for slow tapering. I would try to reduce by 12.5mg at a time, staying at the new dose for a few weeks. I just chop tablets up. I’ve no experience of liquid Sertraline.
Some people suggest taking tablets on alternate days but that seems a very bad idea to me and more likely to result in withdrawal symptoms.
I have never had any withdrawal symptoms when reducing v slowly (at one point down from 200 to 50 over many months). But have had to restart as I just need the medication (but that is not withdrawal, just needing the medicine).
I would recommend they ask someone to help monitor them. The GP may do this but I always tell DH too when I am changing my dose and ask him to let me know if he sees me declining. He can often spot that I am not doing well sooner than I do. Maybe you could offer to help with this. It’s good to have an explicit conversation and agree that the helper can tell the “patient” if they think they are not doing well. Sometimes it’s easier to spot from the outside.
Lastly think about external factors, any painful anniversaries, work or college stressful periods etc and time accordingly. My old GP always said don’t reduce in the late autumn or winter, wait for the spring. If your relative is affected by the seasons I think this is worth considering.
The main thing is - do it slowly and be prepared to plateau at a lower dose for a while.