If my girlfriend married a guy over 20 years ago and he used an alias, is the marriage legal?

Assuming everything else was done correctly, yes.

In Utah, a marriage is *void* only on certain grounds: Utah Code Section 30-1-2. Marriages prohibited and void. The following marriages are prohibited and declared void: (1) when there is a husband or wife living, from whom the person marrying has not been divorced; (2) when the male or female is under 18 years of age unless consent is obtained as provided in Section *30-1-9 *; (3) when the male or female is under 14 years of age or, beginning May 3, 1999, when the male or female is under 16 years of age at the time the parties attempt to enter into the marriage; however, exceptions may be made for a person 15 years of age, under conditions set in accordance with Section *30-1-9 *; (4) between a divorced person and any person other than the one from whom the divorce was secured until the divorce decree becomes absolute, and, if an appeal is taken, until after the affirmance of the decree; and (5) between persons of the same sex. An otherwise lawful marriage can be annulled, however, on certain grounds: Utah Code Section 30-1-17.1. Annulment Grounds for. A marriage may be annulled for any of the following causes existing at the time of the marriage: (1) When the marriage is prohibited or void under Title 30, Chapter 1. (2) Upon grounds existing at common law. Common law grounds include fraud, misrepresentation, lack of intent to enter into a binding marriage, failure to consummate the marriage, duress, undue influence, and physical or mental incapacity. Your girlfriend *may* have a basis for seeking an annulment of her marriage, but whether she could succeed is an open question.