Based on a brief google search, a "fault" divorce in Arkansas requires proof of one of the following:
•a felony conviction
•alcohol abuse for at least one year
•cruel treatment that endangers the other spouse’s life
•behavior which results in intolerable humiliation, embarrassment, or shame to the other spouse
•incurable insanity, and
•willful failure to support the other spouse despite a legal obligation to do so.
If your relative or the spouse is pursuing a fault divorce, they may be looking for testimony about one of those things, either for or against.
A no-fault divorce in Arkansas requires 18 months of separation with no cohabitation or marital relations. So if your relative or the spouse is in a hurry (new relationship, maybe?) they may be pursuing a fault divorce.
In any case you're right, it's best to talk to the attorney, see what they're looking for and tell them what you could or could not testify to, and find out whether you can or can't help before you commit to this.