Since they have the Mp3 deck I assume they have a '08.5 model. The mp3 HU was available pre-'08 as a dealer-only accessory but didn't come standard on any model. As part of the '08 mid-year refresh all HU's became Mp3 capable.
Technically speaking there's a slight pin difference between the two.... I believe the main harness went from a 32 pin to a 30 pin connector. That said other members have successfully swapped between pre/post refresh HU's without an issue.
One thing to keep in mind is the '08.5 one has additional ports on the back of the HU so it's possible to hook up the ACI, and ipod module, and sirius vs. the older model which can only have sirius or ipod but not both. If you don't use any of those then it shouldn't matter.
Other than that like tddvrrn noted as long as both are bose, or both are not bose there won't be an issue with the swap.