Talesin and Euphanism may be right. She may not have realised what she was doing was wrong for a JW to do, and $400.00 to her is like 40 cents to an average joe.
The Flock book on page 136 says:
Christians should avoid gambling in all its various forms,
including lotteries. (Compare Isaiah 65:11; w89 7/15 p.
30; g82 7/8 pp. 25-7; g81 11/22 p. 27.)
- Gambling can lead to or incite unchristian traits, such as
greediness and covetousness. (Rom. 13:9, 10; 1 Cor. 6: 9,
10; Col. 3:5) - Additional bad results from gambling and association
with gamblers give true Christians further reasons for
avoiding it. (1 Cor. 15:33; g85 8/8 pp. 22-3; g82 7/8 pp.
25-7; w80 9/1 pp. 29-30; w72 10/1 pp. 593-4)
If a person makes a practice of gambling and after
repeated counsel unrepentantly pursues a course of
greediness, he could be disfellowshipped from the
Christian congregation. (w80 9/1 pp. 29-30; w67 6/15 p.
356) (italics mine)
Serena didn't make a practice of gambing, it appears it has not been determined that repeated counsel be administered to her, and it says the person "could," not "should" be expelled from the congo.
If indeed she's not baptized, she wouldn't be setting a good example for young JWs, but she's already not doing that. She likely didn't know what she was doing, her bodyguard put up $400, she won, still didn't know what to do, and gave it away. People running in fast crowds can end up in places they wouldn't have dreamed of seeing as a kid.
Also, the elders have a way out of pressing matters:
***
w80 9/1 p. 30 Questions from Readers ***It can be ‘the thin edge of the wedge,’ and especially would this be dangerous in encouraging the gambling spirit where children are among the players. Why not just keep a pencil score, or otherwise play the game simply for the fun of it? The spiritual elders in the congregation would not want to get involved in what you do in this respect. They would not try to legislate as did the religious leaders described at Luke 6:1-5. (italics mine)