I will make two assumptions. I will assume that said person has already accepted he or she may be holding his group's progression back and has made it clear he or she does not mind being replaced for the specific fight in question. I will also assume that no replacements are available.

In this situation, the very best one can do is try to improve one's spec and gear. If he or she doesn't have the very best gear available from Zandalaris/Farm Tiers, then he or she should make it a priority to get that gear, if possible. He or she needs to look at their gems and enchants and make absolutely sure that they are optimal. He or she needs to look at their spec. Is their spec capable of as much damage as another, but is harder to play? They should consider respeccing. Case in point, Frost and Fire are both within a reasonable margin of Arcane's simulated DPS. However, niche fights notwithstanding, Arcane can pump out high numbers with absurd ease compared to Fire. A player with a disability needs to play the easiest spec on progression fights. It's just courtesy.

If all speccing and gearing issues are taken care of, and their DPS is still too low, bearing in mind the above assumptions, you are at an impasse. You can only hope for a better player to come along at some point in the future and fill in for that fight so you can progress.