The christian retreat doesn't need to be a church get together so like YMCA it could be viewed as a conscience matter, I know back in 70s or 80s wts had that QFR article discouraging YMCA but I don't think they follow it strictly as I know many who went to YMCA while I attended the meetings not that long ago. The so called Christian retreat could be a campt or anything social sponsered by some "christian" organization which depending on what it is may not have any implications for a jw unless some congs are really anal about things like that but the congs I went to were not.
I too don't believe in imortality of the soul. The 144k doctrince that teaches the so called anointed will be transformed in a twinkle of and eye to their heavenly reward also doesn't cantradict the bible teaching as in theory the chosen ones are to recieve their reward and be transformed into the spirit realm as spokan in Thessalonians if I remember corectly.
If you want to talk about anything with her talk about 1914 and 607. The whole religion's foundation is based upon 607 as without that date 1914 has no merit for anything even though Charles Russell didn't teach 1914 as what wts teaches it to mean today. If she can't prove to you 607 from any other source besides wts there is no point of trying to prove wts is a true religion. You can ask her why they taught 1799 was beganning of the last days and now they teach 1914 is the start of the last days? How did they realize they were wrong before? As 1799 had nothing to do with them calculating the 6000 years that they suggest for changing the date of return of Christ from 1874 to 1914. Their so called 100 year error still doesn't make sense as one would think they would have moved 1874 to 1974 but they've kept 1914 and pointed to the end of the world in 1975 :) WT dates should be your topic of study IMO from which she'll learn a lot of old wts history and come to realization that she got involved with a fanatical religion based on stupidity of the leaders.