does he have to provide specifics of encounter to win this case.
Providing specifics is not decisive. I will assume you have in mind a jurisdiction in the US.
Since the plaintiff is a celebrity, under US defamation law she would be considered public person. This requires her to prove that any false statements were made with actual malice, which is the term for denoting that those statements were made (1) with knowledge that they were false, or (2) with reckless disregard of whether they were true.
The matter also involves issues of parties' credibility. A defendant might provide plenty of specifics and still fail to persuade the fact-finder (typically the jury) that the statements are true. Conversely, a defendant is likely to prevail over a plaintiff who is known as liar or gives inconsistent testimony.
A plaintiff whose reputation (in this case, on matters of chastity vs. promiscuity) prior to the defamatory statements is bad enough would be considered "defamation-proof", implying that the defendant's statements cannot significantly worsen the plaintiff's reputation.
The import or context of the statements might outweigh the sexual connotation that is typically associated with the expression "slept with X". In that case, the statements, even if false and defamatory, they would not be defamatory per se. This means that the plaintiff would have to prove that the defamatory statements caused her a particular harm (the term for that is special damages).