Siman 365

One who digs a niche for a deceased person, even if the deceased is not his relative, is exempt from the recitation of Shema, and from the prayer, and from tefillin, and from all the mitzvot mentioned in the Torah (Berakhot 14b), according to the law that one who is engaged in a mitzvah is exempt from another mitzvah, even if the time for the recitation of Shema will pass. And even though the recitation of Shema is a passing mitzvah and the burial of the deceased is not a passing mitzvah, and why should he not recite Shema and afterwards finish the niche? However, regarding all matters of the deceased, we do not say this because of the honor of the deceased, for once he has begun to engage in his needs, he should not stop for the sake of another mitzvah.

And that which we say in the Gemara there, that when there were two and the time for the recitation of Shema arrived, one ascends and recites Shema and prays, and then this one returns and digs and his fellow ascends and recites Shema and prays—truthfully, the Tur and the Shulchan Aruch wrote in the name of the Ramban that this is only in a niche where two cannot dig it together. But in a grave where two dig it together—neither of them shall stop, see there. And so they wrote later in siman 403 (Shulchan Aruch Yoreh Deah 403) regarding the gathering of bones, that even if there were many gathering—they shall not stop for any mitzvah. And it is all for the reason that we explained. And it seems to me that it is not specifically regarding the digging of a grave, but rather the same law applies to all the needs of the deceased when one is occupied, if he is compelled to occupy himself with this, such as when the onen is unable to do this thing, or if there are no mourners here—the one occupied is exempt from all mitzvot. However, it seems to me that all this is when he is occupied for free for the sake of a mitzvah. But when he comes for his wage, such as the gravediggers who take payment for their exertion—they are not called "those occupied with a mitzvah" such that they would be exempt from another mitzvah, for they are not occupied for the sake of the mitzvah. However, if it is impossible without them, in that only they are able to occupy themselves with this and not another—they should not nullify their work because of another mitzvah, because of the honor of the deceased.

We have learned in the Mishnah at the end of the second chapter in Bava Batra: One must distance graves from the city fifty cubits, for the reason that a foul odor should not rise to the city. And so it is in Hoshen Mishpat siman 155. And the Levush wrote a reason: so that they do not encounter the graves and become saddened, see there. And his words are perplexing. (And I have seen one who exerts himself to settle his words, and this is even more perplexing. However, in several places there are graves within the city, and perhaps because they had no other place.)