Sean,
There are a few issues he'll have to consider:
1) workstations
If you have Ultras or Eclipses with a hard drive you'll probably be ok getting another POS to run on them. They're just windows PCs.
2) printing
You may have a problem with printing though. Every printer, even the kitchen and bar remotes, are run from the workstations, not the server. The workstations have an RJ45 port configured as COM4 which runs to an RJ11 port on the printers. (The wiring is straight through, pins 1 & 8 aren't used on the larger ends.) Printers are given a line ID that allows up to 8 to run daisy-chained off each workstation.
Some systems use printers with a standard 9 => 25 pin cable from COM1, but they still use the COM4 line for remotes.
3) the new software
Some POS systems already have procedures in place for installing and configuring their software on Micros hardware. Your friend should find out about this and go with one of these companies. When I worked for Micros a few years ago we had a bunch of Aloha and PosiTouch customers who wanted to switch to the 3700 but didn't want to replace the hardware. One of my projects was to get the 3700 to run on non-Micros hardware so I can tell you from experience that if the new system hasn't done this before your friend is going to be the guinnea pig in an ugly science experiment.
4) service
I'm pretty sure the NY Micros office offers a hardware only service contract for people who are using Micros hardware with other POS software but that's probably not the norm. This stuff takes a beating, especially in a bar, so make sure he can get it fixed if it breaks.
Get details on maintenace contracts, like will there always be somebody available to help out if you can't determine if a problem is hardware or software. The last thing you want is to be down on New Year's Eve because the guy who knows how to fix your problem had a few too many and is sleeping it off.
Pat