As a general rule, if someone has an affair with a married individual, she should brace herself for an appearance in divorce court. Ultimately, however, it comes down to the rules in your particular state. If you live in one of the jurisdictions that does not recognize fault grounds, the issue of adultery is often moot. Otherwise, if your spouse files on grounds of adultery, she must prove her allegations through testimony and other evidence.
Fault vs. No-Fault Proceedings
Because no-fault divorce is much easier than proving fault grounds, testimony from a lover may not happen -- particularly if there's no advantage in proving adultery for the wronged spouse. For example, in New Jersey, adultery typically doesn't affect alimony or property division, so a lover's testimony at trial might be pointless. If you live in Virginia, however, issues of adultery can affect a claim for alimony, so it might be worth it for your spouse to prove the adultery and have your lover testify.
Service of Process
If you had an affair and you're now facing divorce, your paramour will probably know relatively early on in the proceedings if your spouse has implicated her. Some states, such as New Jersey, require that she be served with a copy of the divorce complaint if your spouse files on fault grounds of adultery, and the complaint must name her as the individual with whom you committed adultery. Your lover usually has a right to respond to the allegations. She can file her own pleadings in response, denying the charges. A denial may not save her from being called as a witness at trial, however.
If your spouse files for divorce on grounds of adultery, the burden of proof is entirely on her. She typically does not have produce direct evidence, however. Circumstantial evidence is acceptable in most states. She must usually establish at least two things: that you had the inclination to cheat, and that you had the opportunity to do so. Your lover's testimony can be used to corroborate or bolster your spouse's circumstantial evidence.
Divorcing spouses have the right to call any witness at trial if the individual possesses information that is pertinent to the case. Courts will generally agree that your paramour possesses pertinent information. Your spouse would have to subpoena your lover and provide her name as an anticipated witness prior to trial. Some state courts put more weight on a paramour's testimony than others. For example, Mississippi is willing to accept the testimony of a paramour as stand-alone proof.
Other acceptable evidence that corroborates or complements a paramour's testimony can cover a wide range. Your spouse might present photographs of the two of you snuggling in public, emails, text messages, or even the testimony of other witnesses who saw the two of you together. When it's all added up, this evidence may successfully prove adultery grounds.