Does it specifically say tantric sex or tantric yoga? Tantra envolopes many things, and only part of it has to do with sex. Tantra is a practice of liberation, and sex is one means (but not the only means) to achieve this. It is quite common in the west to think of sex when thinking of tantra, but you must be careful.

Maybe try and find out exactly what this tantra practice considers? Maybe you will find it has nothing to do with sex. If it does have to do with sex, find out why she is doing it. Maybe she is unhappy with her sexual life, and is trying to find a way to improve it. You should get the full story before you do anything.