Nullity is a matter of what happened when the marriage was being contracted - i.e. all the grounds matter for the point of the wedding. The common name “annulment” is tricky because it makes people think a marriage can be made void if something bad happens. The truth is that a marriage either is or is not valid from the beginning. A person who willingly took the risk of infecting his or her spouse with an STD may have been immature, not understanding or willing Christian marriage and so on. Also, infidelity and wish for divorce are signs of a serious problem with the goods of marriage (faith and sacrament respectively) and while they of themselves don’t indicate marriage as null, they are clues that problems might have existed already when marriage was being contracted. If a person excludes fidelity or indissolubility at the point of wedding, then the marriage is invalid. Your friend needs to talk to the Tribunal.