Kate, I'm not sure where you got that information about a "sister" being DF'd. The answer according to the WT is as follows:
NO, not for simply marrying an unbeliever. Of course fornication is seen as a possibility if dating a filthy, sexually depraved "worldy" person, hence the panic. If they can avoid fornication before marriage there is no "SERIOUS" sin. As the Shepherding the Flock" book said:
"Marrying an unbeliever is contrary to Bible principles. (Deut. 7:3,4, 1 Cor. 7:39, 2 Cor. 6:14, 15.)
If a dedicated, baptized Christian marries and unbeliever, this would result in disqualification from all special privileges for the time being. (1st Tim. blah blah blah)"
Of course Elders can do what they want but they would have to find something other than dating and marrying an unbeliever in and of itself. Somehow they might invent something to DF your friend for if they don't like her. The "brazen conduct" would not automatically be in play here if she simply marries the guy. She has the right to choose her mate even if it is seen as an unwise choice. She followed God's law in getting married. They cannot trump her right to choose a marriage mate considering she is going to reap the consequences of her choice to "unevenly yoke" with an unbeliever, not them.