I suppose that the axe hanging over his head, i.e. at any moment ICE can swoop in and deport him, could be an issue if you are his client or his partner and you are exceedingly paranoid. Most attorneys do not have adequate procedures in place for how to handle their caseload if they die or become debilitated. Unfortunately he would have to make a similar plan, but the contingency being apprehension by authorities rather than death or disability. Even though this is extremely unlikely as Forma pointed out.
I also suppose flying anywhere on a plane is also an issue. My undocumented friends cannot fly. Maybe a pro-hac vice admission in another state could be problematic. I also suppose it might be nerve-wracking to practice in Federal District Court, but it would really take an enterprising and gung-ho Joe Arpaio type Judge or Federal Marshall to do anything to him. But if you want, you can avoid federal court for your whole law career. Really the only day to day issue would be paying him and how to deal with the taxes. But these days even that can be accomplished in an above-board fashion with IRS ITIN's.
So while his life will be complicated by these things, he has probably dealt with this type of adversity his whole life and he can certainly find a niche practice where he could do very well, without his status ever being an issue.
I also suppose flying anywhere on a plane is also an issue. My undocumented friends cannot fly. Maybe a pro-hac vice admission in another state could be problematic. I also suppose it might be nerve-wracking to practice in Federal District Court, but it would really take an enterprising and gung-ho Joe Arpaio type Judge or Federal Marshall to do anything to him. But if you want, you can avoid federal court for your whole law career. Really the only day to day issue would be paying him and how to deal with the taxes. But these days even that can be accomplished in an above-board fashion with IRS ITIN's.
So while his life will be complicated by these things, he has probably dealt with this type of adversity his whole life and he can certainly find a niche practice where he could do very well, without his status ever being an issue.