This is sticky, that’s for sure.
I’m no canon lawyer, but it’s my understanding that under Catholic moral teaching, one may not engage in an objectively sinful act, even if the intended end is for good.
So I guess what we’d have to find out first is, what is the nature of this “Testimony?” Is it an offcial “rite” in the Morman community, designed either to a) encourage, deepen and strengthen one’s beliefs and practices with regard the teachings of Mormanism, or b) attempt to persuade onlookers to accept some aspect of Morman teaching, or c) an informal observance whereby one extols the work of God in their life.
It would seem to me that both a & b above would be “No Sir-ee, Bob!” time, as the purpose is to either further one’s commitment to the LDS organization or to encourage others to consider Morman teaching. Mormanism is an objectively malformed belief system; originally a hybrid of 19th century low-church protestantism with a fair amount of freemasonry thrown in the mix. It is also materially heretical, although it may not be formally so (insofar as the individual holding Morman beliefs may not realize or understand that many of their teachings are in obstinate opposition to the Catholic Church’s doctrines.) Consequently, if there’s any purpose to the practice of this Testimony to either confirm the individual in the Morman faith, or to “proselytize” others, it would seem to me to be decidedly verboten.
Option “c” above is a little more dicey. So long as it is informal and does not attempt to “evangelize” others, it may be permisible provided one gets the go ahead from their spiritual director, who should be able to make an educated judgement call as to the spiritual state of the individual and whether or not placing themself in this environment might be potentially dangerous to one’s remaining strong in the Catholic faith.
Again, I’m not an expert, but that’s my take on it just off the cuff. May God grant you wisdom in how to deal with this situation. I don’t envy you, that’s certain!