Half of the people replying are completely ignoring the specifics of your question; I'll try and help.
I'm not familiar with VZW's BB 8800, but let's assume it's a CDMA/GSM hybrid like you say (with only foreign GSM frequencies, 900/1800). He'll be paying upwards of $1 a minute to use the phone in Germany, and will simply be roaming on some local GSM network Verizon has an agreement with. He won't be able to use two numbers in the phone at the same time, since IIRC Verizon gives you a SIM card to use abroad in their "hybrid" phones; the CDMA ESN (what identifies the phone on VZW's network) does nothing when overseas. The best option is to take a cheap as shit unlocked GSM phone over there and buy a prepaid German T-Mobile SIM when you get there; watch out for the Motorola V195 deal, T-Mobile USA only unlocks your phone for you if you've been a customer for 90 days. If you have any T-Mobile friends, have them call it in as if it is their phone.
Cliffs:
If he wants to use the VZW phone, it will ONLY function with the US number, and will be expensive as balls
Carry a cheap GSM phone with a "Deutsch Telekom" SIM card or both phones, avoid using the VZW phone as much as possible