[ossiblue]

If the cactus can be replanted with good chance of survival, I see two options. First, if you neighbor wants to keep it, give him a hand and replant it and help him secure it from further damage until it re-roots. Second, if he doesn't want it, call a landscaper and ask them if they want it for free. Again, if it can be replanted with a good chance of success, it is something a landscaper might jump at to take off you neighbor's hands.

If there is little chance it can be saved, cutting it up is the best option, IMO, following Zeke's suggestion of potting some cuttings for future use/sale. Aren't saguaro fairly expensive to purchase from a nursery or landscaper?