Buddhist beliefs about death are categorically different than Catholic beliefs. Buddhist chants are not appropriate in a Catholic church, or as any part of a Catholic funeral, or at the graveside. I would be seriously weirded out by Buddhist chants at the funeral home or at the house, but at least that would be a secular setting. You could make an argument for non-Catholic chants outside church after a funeral; but really, that would go against Catholic tradition too, because the early Christian tradition was for all the funeral attendees to be singing hymns the whole time during the funeral procession to the grave.
I found out the probable content. In Thai Buddhism “the custom is to recite the mātikās to the seven books of the
Abhidhamma Piṭaka, a practice that’s aimed at the benefit of the deceased rather than the people attending the funeral. The underlying belief is that the spirit of the deceased is hanging around the coffin and needs to be urged to go and get reborn. And so the chanting of the Abhidhamma is aimed at informing the spirit that the rūpadhammas that make up its corpse are no use to it anymore and so it’s time to let go of attachment to it and move on.”
Catholics do not believe in reincarnation rebirth. They do not believe that the soul abandons the body, but rather that the body will be reunited with the soul and resurrected on the Last Day. They do not treat the body as some kind of worthless husk, but rather as a holy thing created by God and destined for immortality. And we sure as heck do not agree with the Abhidhamma Pitaka’s “theology” and arguments.
This was not an ecumenical gesture. This was the priest saying, “Okay, I’m cool with a corps of non-Catholic religious figures, formally denying every tenet of Catholicism and promoting their own beliefs, in church, right in front of the deceased who is hopefully a saint or a saint in Purgatory. Because really, Catholicism isn’t true and doesn’t matter, so it’s all the same thing as long as the widow is happy and it’s purty moozick.”
That said, I’m sure the priest didn’t realize what the monks were going to sing, and he probably was just trying to be nice and accommodating. And the widow may or may not have realized how much she was violating her husband’s beliefs; heck, the husband may not have objected in life, if he was not a very savvy Catholic. But it was not right, even though it could have done no harm to the deceased.