The laws can vary significantly between states and countries - your use of language suggests that you are in the UK? Over here, a court is more likely to consider such evidence if there is also evidence that the infidelity contributed to the breakdown of the marriage - and if another affair from around the same time is known, it is more difficult to gauge what, if any, effect the unknown affair had on the breakdown of the marriage.

Given the importance of local law to this type of issue, I suggest that your friend consult a local lawyer about her plight, and see what they suggest.