Officially it is a No No. There was a JW org video recently on a JW married to a "worldly" person dithering as to whether she would go to a church ( I think to a wedding. ) The Society left her final answer blank , but strongly implied that she wouldn't go. Generally , in my experience , JWs normally would go to a church if they don't have much of a choice ( eg for a funeral or close relative's wedding ) to keep the peace but pretty much keep it quiet.
I still remember my grandmother's funeral 30+years ago when my parents refused to attend so we hung around outside the church in freezing weather waiting for the service to finish. It was pretty embarrassing and it didn't go down well with the ( non JW ) family.
What is strange is that the Society don't have any problems selling off Bethels to Christendom ( such as the New Zealand one ) so, as ever , there does seem a lot of hypocrisy here.