ugh, I hope you can answer him "yes". heh-heh.
Matthew, you would have to support everything around the girder. The floor joists, the end of the girder that you are going to leave,,maybe more. Then you can cut the rot out. SOMETIMES, you will find that the girder is rotten down the center between the 2 bys and the repair grows. Be prepared to replace more than you think and to stagger your splice with 2 bys cut at least 2ft longer/shorter, kind of finger jointed together. A stack of 2 bys butt jointed (all cut to the same length) will not have much integrity and could possibly creel over or start to separate and spread and could lead to even more problems. After the splice is made, support both ends of the splice and double check the pockets in the foundation walls. make sure that the wood is not in direct contact with the concrete. Tar paper is better than nothing to separate the wood from the concrete. On the support piers, use steel shims on top, not wood. Pour a footer pad and use 8" or wider masonry blocks with mortar between. The mortar will distribute the load across the blocks evenly.
It IS a BIG job. I personally hate to do anything to a girder after it's installed.
Tom in KY, Do not ignore InspectorD's post. Check it all out. A home inspection before you buy is not a bad deal.