One issue that does come into play is the question of "for hire". Since the FAA has ruled that flight time is a form of "compensation" there is definately grey area if you are just logging time for the purpose of logging time. Particularly as a private pilot. If you can show that you are conducting legitimate training with the end goal of taking a multi-engine checkride (presumably in that particular type of aircraft), then I'd say go for it. (Including the XC time).
However if the person you're flying with doesn't have an MEI or would be willing to let you fly from the left seat from time to time, I'd probably pass up the opportunity of logging the time.
Either way, be sure to take them up on the opportunity as a learning experience. There's definately no regulation that says you can't fly along and enjoy the ride (as well as put some systems and procedures knowledge into your grey matter.)