Ah. Mercalli doesn't work with Premiere 6 so your friend would have to move to (the next?) version of Premiere.
Yes, Edition 4.5 was a bit of a techie's beast. I had a copy given to me when I met the development team in Munich (I was invited over there to 'chat' to them about making Edition more usable for Studio users - an almost impossible task at the time!). Flatteringly, they did take on board a lot of my suggestions, as seen in later versions, but I could never really get to grips with V4.5 at all. So I can understand getting rid of it in favour of something else (anything else would have been better, usage wise!)
The new Liquid (Liquid-ate? No, that's one reason why they're changing the name!) will unquestionably (I sincerely hope!) have tbe same time-line power as the current version. That is
both in line and multi-track A/B rolls. If you prefer A/B rolls and twin-line edits - you've got it. In fact you can have as many time-lines, each dedicated to exactly what you choose (Video+audio, Video, Audio stereo, Audio 1, 2, 3 or 4', Audio with scrubbing, Audio without scrubbing ... locked, unlocked, active, inactive etc etc etc). And you can use those time-lines in whichever way your editing preferences desire.
In-line editing in Liquid for transitions is far more powerful than you might imagine. Unlike programs like, say, Studio, a transition doesn't sit equally across both clips. In fact, it is exactly the same as two-line (A/B roll) transitions - but if you can imagine it, with both lines fused into one. With two line transitions, the two clips must 'overlap' for the transition area. This is also true of the Liquid single line transition - the 'overlap' is called 'leeway' in Liquid parlance. If that 'leeway' isn't there (leeway means material between the clip begin or end marker, and the actual clip's beginning or end), then this is indicated in the transition graphic. This method means that a clip with 'no' leeway - ie you want to use every fraction of the clip - can still be used in a transition - as long as the 'other' clip in the transition has the required leeway! There is an article about this on my webpages - aimed at Studio users upgrading to Liquid, but still shows what I mean (I hope) -
http://www.mikeshaw.co.uk/editionforstudio.htm#usetrans
It was written for Edition 5 & 6, but is still relevant on Liquid 7 - and I'm sure, all fuiture versions.
However, as I said, if you prefer working on two timelines, then you can. The beauty of Liquid is it lets you tailor - and/or select - the layout you want, and work exactly the way you want. There is even a 'Premiere' user detox, and a Premiere user type interface and controls set to help make the transition from Premiere to Liquid. When I watch other people using Liquid and see the way they work, it is never the same as me - and often, I see them doing things quicker and easier 'their way' than my way. So I learn and change... I just hope the new version will maintain this flexibility. Knowing the Munich team, I'm sure it will. It would be disastrous to step back on capabilities - they're more likely to move forward I think.
Hope all that helps!