Hm. I can't help with the Jabber thing, but you are facing some challenges in going cross-platform (Mac->PC) and cross-chat-client (iChat-->GoogleTalk).
Don't suppose you two would consider using Skype
instead? It's rock-solid, and Skype-to-Skype calls are free. (Skype-to-landline calls are really cheap, too - that's what I use to keep in touch with my folks back in Canada, I'm in Mexico).