I just went through this with my uncle (non-JW) and his brother, my father (JW). My father attended the viewing, but left before the service, which was held in the chapel area of the funeral home. My dad didn't attend the interrment either because my uncle's reverend was going to be saying a few words there too.
I know exactly what you're going through. Is their faith SO weak that hearing some other belief is going to stumble them? Are they afraid Satan lives in churches and will somehow possess them? You would think going door-to-door where people (like me) will actually attack them about their beliefs and leave dumbfounded looks on their faces would be more of a concern than hearing a reverend (,pastor, minister, priest, etc.) talk from a podium. It's not one-on-one. It's not back-and-forth. He's been a JW for over 50 years. What is that going to do.
I looked up some articles. Seems everything I found says it's a matter of your conscience. They talk of dangers and put out all the warning of why you shouldn't go. But, basically say it's up to each person and to be careful about what they participate in. I know, I know, in JW-world, warnings are as good as an iron-clad ban on the issue.
Watchtower 3/15/70:
Some Christians may feel obligated to attend a church funeral because of a debt of gratitude, because a close relative is involved or due to pressures from an unbelieving mate. But before doing so each one should consider the various factors involved and the possible alternatives. While doing so is not forbidden by the Christian congregation, such a course is certainly fraught with dangers and problems.
Watchtower 10/15/52:
What about attendance at a funeral conducted by a clergyman of one of the many false religions? We do not advocate it as a general practice, for such a practice would be very unwise. However, there may be circumstances when those in the truth would attend such a funeral, and could do so without being censured for it. Close relatives, who for reasons beyond their control feel obliged to go to a church building of some false religion to see their loved one buried, go there to see the funeral and perhaps comfort others in attendance. They do not go there to practice false worship, if they are in the truth. It is the same as in the apostle Paul's day in the case of a man who went into an idol's temple for something to eat. He goes in there to get a meal, but not to worship. (1st Cor. 8:7-10, NW) Some other brother's conscience would not be strong enough to permit him to do this, and his weak conscience would take offense if he saw his Christian brother in such a place for just a meal. So while we do not advocate attendance at clergy-conducted funerals, it is not within our province to criticize or condemn, but we will let God judge our brother who according to his conscience may feel obligated to attend functions under clergy officiation.
