Although you have more TID information provided by your Explorer than she does the X-70, there are a lot more similarities in those detectors than one might think. Especially if she is using a small DD coil. My recommendation would be to actually hunt together. In other words, work an area side by side and do a comparison of the target signals. If you get one that you think is going to be a good target, let her sweep over it before either one of you dig it. Explain why you think it is good on your Explorer and see what differences there are between the two detectors on the same target. Likewise, if you get one that sounds like it is going to be a piece of junk, unplug your headphones, both of you listen to it, and explain why you believe it to be bad. Then, dig it anyway and prove (or disprove) your diagnosis. Likewise, have her let you know when she sweeps a target that she is planning to dig. But before she digs it, you listen to it with the Explorer and then listen to it on her X-70. Again, let her hear the Explorer sounds and explain what it is you are hearing that she may not be. Trust me, there is a lot more similarities in the audio response of the X-Terra and the FBS lineup than most would think.
I spent all summer learning how to use the Etrac effectively. After having a couple Explorers in the past, I guarantee you that I would not have been as successful with the Etrac without having used the X-Terras (multiple tone mode) for the past 4 years. In the same maner, the Etrac taught me things about the X-Terra that I had either overlooked or taken for granted. Just as the X-Terra made me a better hunter with the Etrac, I can honestly say I am a better X-Terra user today, due to using the Etrac this summer.
Point is, the audio will sound similar with both detectors. Listen for solid hits, no breakup in audio and when you drag the coil back from the target, dig those whose signal drops off suddenly. If it "bleeds" away and the audio gets a much lower pitch before the sound disappears, (all metal mode) it probably won't be a keeper. And if she isn't using a DD coil, get her the 6-inch one. Heck, you're using a DD coil and you know it separates better than a concentric. Ain't any reason she shouldn't have one! JMHO HH Randy