While it's probably worthwhile for your friend to discuss the situation in depth with a personanl injury attorney, since every case has its own complexities and unique issues, as a general matter, no--your friend could not sue his employer for two reasons:1) First and most importantly, Worker's Compensation, when an employee receives it for a work- or workplace-related injury, is generally the only recourse against the employer; the employee gives up the right to sue in accepting the Worker's Compensation. The trade off is that while he cannot sue, he receives compensation without the cost, delay, and uncertainty of litigation.2) Generally, there is no recovery for mental anguish when the accident is the result of simple negligence, or carelessness; there usually has to be something more, like gross recklessness or an intention to cause injury/suffering, to support this sort of compensation. It is unlikely that the necessary bad motive or recklessness exists here--cases like this typically turn on simple negligence.However, it may be possible to sue the oxygen tank manufacturer, if there was anything defective in its design and/or manufacture, for losses or injuries which exceed the amount of Worker's Compensation he's receiving; this is something it would be worthwhile to discuss with the personal injury attorney.