A catholic wanted to have her baptism blessed under full water method. The Church is not familiar with this , so she had a evangelical church do it. So, her local Catholic priest forbid her from receiving communion. She was not aware of the consequences of minor excommunication. Now, what can she do to go to Communion again?
We don't have all the information to be sure what is going on, but here are some likely scenarios.
The Catholic Church has no objection to baptism by immersion. It's a completely valid means of baptism, and Catholics do understand it. However pouring and sprinkling are considered equally valid. Moreover most Catholic churches are not equipped for immersion baptism. If your friend specifically requested baptism by immersion then the priest may have declined out of practicality.
Additionally the Catholic church considers baptism a one time thing. If your friend was previously validly baptized then the priest will have declined to baptize again. Doing so would be a sign that the first baptism was invalid, which would go against fundamental beliefs.
The issue therefore was likely not being baptized by immersion, but deciding to get a major events in their Christian Life done in a church other than the Catholic one. This is not really consistent with a full commitment to the Catholic church, or a belief that it is the best church. Getting baptized in another church will have been taken as a decision to join that church , and by implication leave the Catholic church. The other church will have taken it that way. Joining another church makes you ineligible to take communion in the Catholic church.
If this immersion baptism happened a long time ago and your friend now wants to join the Catholic church, that should not be a problem. She will have to go through formal acceptance into the Catholic church, but that will likely not involve another baptism.
If your friend truly did not understand the implications of what they were doing then they should talk to their priest to sort it out. However that is very unlikely. The other church will likely have insisted on some instruction before baptism, and that will have included implications for membership. They will also have assured themselves that your friend follows Evangelical beliefs, which would be different from Catholic beliefs. The Catholic priest would certainly have explained the implications if the question had come up.
"Full water method" sounds like a full immersion, but is there something else perhaps that the Catholic Church would not recognize in this baptismal type?
Otherwise, she (Catholic Church) says this about baptism. It is:
"The sacramental bond of the unity of Christians
1271 Baptism constitutes the foundation of communion among all Christians, including those who are not yet in full communion with the Catholic Church: "For men who believe in Christ and have been properly baptized are put in some, though imperfect, communion with the Catholic Church. Justified by faith in Baptism, [they] are incorporated into Christ; they therefore have a right to be called Christians, and with good reason are accepted as brothers by the children of the Catholic Church."81 "Baptism therefore constitutes the sacramental bond of unity existing among all who through it are reborn."82
So, unless there's some other problem, like LDS baptism, (different Christ), then not sure why the priest wouldn't recognize the baptismal community of all Christians.
If the baptism followed the liturgy and simple requirements found in the Didache, which is recognized by the Catholic Church as the oldest known example for those in the faith to follow, then the priest cannot object to it. The Didache outlines a baptism in cold, free-flowing water with a full immersion to one that consists of the sprinkling of any water available.