Here's a thought:
Your mother seems very close to this widower, right? Do you think she invites him over for dinner, etc. Could she be spending more than the allotted hours alone with him? Is she always chaperoned? Your father could maybe turn the tables on her and bust her and confront her with it. It matters not to the elders whether your mother was just having tea and crumpets and innocently playing footsies under the coffee table. All they care about is that she was alone with him for such and such a block of time.
I have been put through this same question barrage by my exhusband. What a bunch of weasels JWs can be when they want out of a marriage very badly. I don't have any sympathy for people who divorce for convenience sake and then whine to know if they are free so they can complete their evil plan which is to remarry the perfect JW mate and live happily ever after.
If widower is serious about marrying your ma, then let him go get some extra offices to clean or a second job and let them pay for a PI to get the goods on your dad. The way I see it is your mother has enough evidence. If I were her I'd just say to the brothers, look my ex has had several girlfriends. I am confident he has had sex with at least one of them. I'm getting married. Enough said. That's it. What is the worst that can happen to your mother? She gets dfd and then has to be reinstated? Does she not know her Jehovah is watching her and he knows she divorced your father for bogus reasons and will be held just as accountable for his adultery?
Weasels. Just a bunch of sleazy weasels.
Heather