My comment: Get an engineer who is familiar with the loads and materials.

Or, because you are not the owner, make an offer contingent on the owner fixing the problem.

"4 jack posts to distribute the wieght"

Careful of overloading the floor. Even a concrete slab may crack. It is not a foundation footing.

This is the sort of job a person better know what they're doing without question. You need to have that person present to do the job.

It can be quite a trick to fit a new beam into an existing space, without taking out some wall(s).

Before trying to repair the beam you need to address the source of the moisture that led to the beam rotting.

It is rarely simple as adding new wood and posts, the posts need a suitable e bearing surface at the floor.

A 3 inch thick slab is NOT adequate (think more like 8-10 inches of footer over a couple square feet depending ion soil type).

It is pretty common to erect temporary supports and then remove and replace the existing beam (with enough load spreading from large timbers the slab can take the loads short term, but you need railroad tie sizes timbers in some cases).

It is NOT a job for the inexperienced.