I posted about my hub on dogs going to heaven in your other question, but I will often use those examples to help ease people's grief. I also always remind them what a blessing they were to their furred family member (if it's furred. :)) and what a blessing that family member pet was to them.
People just need time to heal from the grief. If they had to put their pet to sleep, they need friends to tell them it was the right decision and the best, most loving decision for their pet. Some people do best if they get a new pet right away. Others need to wait some time before getting a new pet.
It's very hard to lose our pet family members, and the grief can be overwhelming. Most larger metro areas actually have grief support groups for people struggling over the loss of a pet. If your friend is really suffering and can't seem to get past the grief, suggest they contact one of these groups. I bet a local vet would know about how to contact such a group.