A potential client might better be referred to as a suitor.
The term lady friend might be a respectable way to refer to a woman, and gentleman friend for a man, without implying any degree of personal intimacy such as boyfriend and girlfriend suggests.
A suitor then, might arrive as a client, and leave as a gentleman friend.
Or, instead of "client," a suitor might be referred to either as a gentleman caller or simply as a "visitor," and THEN leave as a gentleman friend.
I prefer "lady friend" and "gentleman friend" to provider, ATF, or regular, or anything like that, but some men might feel it's too stuffy.
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