If they actually want a religious wedding, I suggest reading the link below. I think it’s a good summary of the rules from a Catholic point of view.
I hesitate to post this, but… when you say that there are Catholic priests who advertize they will perform interfaith marriages, you should be aware that, while that it is possible, it is unlikely. Some of the “priests” who advertise they will do interfaith marriages are not priests and/or are not priests in good standing.
There are some people who act as if they are Catholic priests and perform weddings and baptisms for Catholics who cannot or do not wish to follow Church rules. These marriages are not regarded as valid by the Catholic Church. See https://www.turnto23.com/news/local-news/diocese-warns-against-catholic-ceremony-scam
A variant of this is the “American Catholic Church,” which I assure you is NOT recognized by the Catholic Church. See for example, this wedding in the NY Times. “Antoinette Yeh, a daughter of Mr. and Mrs. Tsun-Cheng Yeh of Ardsley, N.Y., was married yesterday to Marc Justin Genovese, a son of Mr. and Mrs. Ralph Genovese of Avon, Conn. The Rev. Robert J. Allmen, a priest of the American Catholic Church in the United States, officiated at Oheka, the Otto Kahn house in Cold Spring Hills, N.Y.” https://www.nytimes.com/2000/10/01/style/weddings-antoinette-yeh-marc-genovese.html?searchResultPosition=1
Or this gay ceremony, also reported in the Times “Rodney D. Harder and Ray B. Gray Jr., artists and art teachers in New York, celebrated their partnership yesterday. The Rev. David A. Murphy, a priest of the American Catholic Church in the United States, led the ceremony at the couple’s home in Cooks Falls, N.Y.” https://www.nytimes.com/2005/06/26/fashion/weddings/rodney-harder-and-tony-gray.html?searchResultPosition=2
Or another gay “Catholic” wedding
“Dr. Luis Etienne Tollinche and Peter Raymond Meyer were married Wednesday evening at Eleven Madison Park, a restaurant in New York. The Rev. Ed Ingebretsen, a priest affiliated with the American Catholic Church in the United States, officiated.” https://www.nytimes.com/2014/06/22/fashion/weddings/luis-tollinche-peter-meyer.html?searchResultPosition=4
Please note: I am not telling you or anyone else not to make use of their services; I just think many of them mislead people into thinking they are Church approved. Obviously, that’s not the case for a gay couple; they are aware that they cannot marry in the Church. But there are people who think they’ve found a priest who will marry them despite the fact that one of them is divorced and has not obtained an annulment of the previous marriage or the couple doesn’t want to get any counseling.
I’m just trying to make you aware that these “priests” are usually not actual Catholic priests. (There’s one group of men who dress as if they are Franciscans. Slight problem: most of them are married and certainly don’t live in a monastery.)
If anyone in your daughter in law’s family is a practicing Catholic, they might well be more offended by a wedding performed by one of these men than they would be by a civil ceremony.
Again,just making you aware of the issue; not telling you what the young couple should do.