Prostitution is considered a "conditional bar" to establishing good moral character. INA § 101(f) and 8 CFR 316.10.
The USCIS website's help center says, "[a]ny person coming to the United States to engage in prostitution, or any person who has engaged in prostitution within ten years of his or her application for a visa, adjustment of status, or entry into the United States, is inadmissible. This section also applies to those who have made a profit from prostitution."
However, the Policy Manual, volume 12, chapter 5, part F, says that engaging in prostitution once, doesn't fall within their definition of engaging in prostitution. Part F states:
"An applicant may not establish GMC if he or she has engaged in prostitution, procured or attempted to procure or to import prostitutes or persons for the purpose of prostitution, or received proceeds from prostitution during the statutory period. The BIA has held that to “engage in” prostitution, one must have engaged in a regular pattern of behavior or conduct. The BIA has also determined that a single act of soliciting prostitution on one’s own behalf is not the same as procurement."
From that language, it seems that a client won't be barred per se for participating once in prostitution. However, an applicant still needs to demonstrate five years of good moral standing to qualify for naturalization, as found in chapter 9 of the Policy Manual. So hopefully a client’s act of prostitution, when taken into account with their other acts, is not enough for them to fail their good moral standing test.
