Well, there are two points that I can bring up.
Deductible medical expenses are those to treat, diagnose, cure or mitigate any disease or condition affecting the natural function of the mind or body. Medically assisted death is not a cure, mitigation or treatment in the conventional sense (although the person is certainly no longer experiencing the effects of their illness or condition). I think that's going to be tricky.
Also, medical expenses paid overseas are generally deductible, but medical expenses are not deductible if the treatment would be illegal in the US. This is most commonly applied to prescription drugs that are not legal in the US, but I think it would apply to medically assisted dying as well. (I don't know the legal status of MAID in the US or in specific states.)
Some expenses (such as to confirm a diagnosis) might be allowable, I think the patient or tax preparer needs to carefully consider the individual expenses on a case by case basis.
And of course, medical expenses are only deductible for the taxpayer themself, their spouse, or their dependents. So you could not deduct your relative's medical expenses unless they were your tax dependent. They (or their representative) could deduct expenses on their final return or their spouse could deduct expenses on their joint return, if they expenses are deemed deductible.
