As I read the situation, the OP wasn't the first to sign. The other cacher still had the logbook, borrowed the pen and then gave the logbook and pen to the OP. Of course, I may be wrong.
The way I read it, FDNA rehid the cache while she went back to her car to get a pen. In the meantime LewisClan shows up and starts looking for the cache. She let's him find it. If I were him I would sign the log first claim the FTF and then loan the pen to FDNA to sign if she wanted to. I think offering the co-FTF was very gracious on his part.
In her second post, she mentioned that she took the log with her to the car.
If she took the log back to the car, then what he found wasn't a cache since it didn't have a log book