I gather they aren't doing too much cycling currently, and that touring is a medium-term goal; is this right?
The bike she is interested in would make longer rides harder and less comfortable - the riding position is sitting straight up, and I can't see any arrangements for carrying anything on the frame (no eyelets for racks, as far as I can see). The redline 510 looks to be a pretty low-end bike - the components aren't very good - if she decides on a redline, go for the 520 instead.
Re stepthrough frame - it will be weaker. If she is comfortable getting on and off a diamond-frame bike, that might be preferable. Thorn (UK) and Rivendell (USA) make mixte-frame touring bikes but they are quite expensive.
The Kona Smoke is an affordable and OK-quality (not great but fine to start with) bike that would be suitable for touring. There are a bunch of hybrids that would do well.
The other thing to consider would be for the two of you to try a tandem (Hire one a few times, see if you like it). It isn't for everyone, and requires a commitment to teamwork, but if the three of you want to tour this might be the best way for your fiancée to enjoy the trip.