The law never requires severance: it's voluntary for companies to offer it. If there was some sort of severance agreement and its terms address this issue in some fashion--or at least provide some defined basis for receiving severance--those terms will be enforceable. For example, if the agreement says that in exchange for being given severance, your friend gives up any/all potential legal claims against the company, then she should still be eligible for the severance: the severance was consideration (or quid pro quo) for giving up legal claims, and that's still the case regardless of whom she gets  new job with.On the other hand, if there is no severance agreement and nothing that your friend is doing or giving up to get the severance--it's simply something the company chooses to offer voluntarily, to ease the pain of being fired or laid off--then the company may be able to withdraw the severance if she simply moves from one job to another with them.