Other person pretended to be the guardian and signed the marriage contract

My boyfriend is now separated from his wife for 5 years already. Now we are planning to settle all legalities about our relationship. I asked him about their marriage and he brought up that their marriage was a secret one (civil) and they didn't tell any relative of his wife because they didn't like my boyfriend back then and they will not approve. They were just 23 years old at the time of their marriage. In their marriage contract, a guardian's consent was needed. Since there was no one who came for his wife, my boyfriend's uncle pretended to be the eldest brother of his wife and signed the marriage contract. Will it be a cause for their marriage to be invalid since they lied under an oath?

What your boyfriend's uncle did was falsification. Unfortunately for you, no that is not a ground for the invalidation of the marriage.

In the first place, your boyfriend at the time did not need parental consent because he was not between the ages of 18-21 (Where parental consent is needed and the lack thereof is a ground for annulment).

What he needed was parental advice. The lack of parental advice only delays the issuance of the marriage license, it does not affect the validity of the marriage itself.

Your boyfriend is still married.