Generally, a Catholic who is not married in the Church (meaning their marriage wasn't witnessed by a Catholic priest or deacon and two witnesses, unless they received a dispensation) is considered to be in an irregular marriage situation, and therefore,
cannot receive Holy Communion.
However, since you mention they will get married in the Church soon, the situation is temporary and the following applies:
* Before the marriage is convalidated (blessed/recognized by the Church), they are generally not permitted to receive communion. Their current marital state is considered irregular according to Church teaching.
* Once the marriage is convalidated in the Church, they are in good standing and can receive communion, provided they are otherwise in a state of grace (e.g., have confessed any serious sins).
It's crucial for the individual to speak with a priest about their specific situation. The priest can provide guidance, answer questions, and help them prepare to receive the Sacrament of Reconciliation (Confession) if needed, and ultimately receive communion after their marriage is validated by the Church. The specific circumstances can affect the priest's advice.
In summary, receiving communion is contingent on their marital status being in accordance with Church teaching. As they are planning to rectify their marriage, they need to speak with a priest to guide them through the process and advise when they can receive communion again.