Some seats have had problems with the harness coming off the splitter plate. It seems to have almost always come out of the shell in the parents hand when tightening the harness, and is only an issue if the harness is loosened and tightened regularly. Not all seats seem to be prone to doing it - even amongst those that don't have the updated harness.
Britax has a new harness that they're using - they've added some stitching about 1/4" or so from the bottom of the loops that makes it impossible for it to fold in on itself. I'm sure if she had problems Britax would replace the harness.
I expect that Britax will have to release a recall on the harness, but don't know how long that will be. It may be worth her calling and seeing if they'll send a new harness out when she buys the seat and then she won't have to worry about it.
In the end it seems like the risk can be almost completely eliminated by not loosening & tightening the harness with every trip, loosening by pulling on both straps evenly when you do loosen, and checking on a regular basis to be sure the straps are still attached.
The Radian would be another option to provide plenty of height growing room and a 65lb weight limit if she isn't comfortable with the Frontier. It just doesn't convert to a booster...
If she'll never have use for a convertible seat, I'd be inclined to just get a Frontier if the Nautilus isn't here in time for her to have a chance to compare it, and then call about a replacement harness. If she's going to go that route, she may be best off buying the seat sooner rather than later so that she has time to receive the harness before needing the seat. Otherwise she could just not loosen and tighten it until Britax gets everything in order. Personally speaking, I'd rather spend $300 on a Frontier than $280 on a Radian if I wouldn't need the rf'ing capability at some point down the road, even if the issue isn't 100% resolved. (Assuming no other comparable choice of combo seat available at time of shopping. I'm guessing she's ruled out the Apex?) I would be inclined to not use it until the issue was resolved as long as her child has growing room in his/her current seat, just because that's my own personal comfort level, but if she needed it before then, and was careful with it, there's no inherent danger in using it if she follows the precautions given above.