If you feel you must warn her Jim, you might want to tell her that you are an ex-JW and JWs are taught that those who leave their religion are scum, possessed by Satan and are to be treated as they were dead, under all any and all circumstances. Since she already knows you are not like that, it should set off a lot of reg flags immediately.
Since she already has a deep faith in God, she's probably quite aware that Jesus hung out with people the Pharisees thought were outcasts and scum, too. In telling her that (if you decided to do so) you could also tell her that you haven't mentioned your JW status to the co-worker because 1) you don't want her to begin to treat you like pond scum, and 2) it could disrupt your work and working relationship with the JW. You are only interested in keeping the peace.
You could explain in detail how JWs are taught by their Brooklyn Masters to deal with people who leave their faith and how it destroys families, marriages and friendships. They treat JWs who ask pesky questions about their religion, or smoke cigarettes exactly like people who are serial killers. No exceptions. It's not that difficult to get in, but can be nearly impossible for many to get out. Once in the JW religion one has NO freedoms, including the freedom to think and question differently from what they think and teach. Did Jesus want his followers to feel hopelessly trapped and blackmailed and extorted into staying his followers?
Then if the dub does call, the mother will have some interesting questions to ask her about how JWs deal with those who leave their faith. Of course, it would also be good to first show some WTS documentation that proves beyond any doubt that what I've said is true. If the dub tries to skirt around the issue or lies about it, your friend will immediately see hypocricy and deceit. If the dub admits it, you friend can see the the JW religion's evil for what it is. Either way, the dub won't likely trap another victim.
Then there's always the message of the "Good News" you co-worker has to offer: 6 million happy JWs stumbling around over 6 BILLION slaughtered non-JWs. That always gives me a nice warm fuzzy feeling whenever I think about it. :D)
Farkel