Proper procedure when promised funding not given to Ph.D. graduate student
That "contingent on the availability of funds" is indeed an escape clause for the university, such that they technically have no responsibility to ensure funding.  

However, it is entirely unreasonable for a Ph.D. student in a STEM field in the US to not be paid a living wage for their work.  Your friend needs to make the department aware of this situation, as departments often have resources that can be used to help to cover small gaps.  Now it's possible that neither the department nor the professor is able to obtain the necessary funds, in which case, I would recommend that your friend consider seeking paid employment in industry and finish their Ph.D. part-time.

Having a student do this would be quite embarrassing to most professors, so making it known that this is a possibility may help ensure that appropriate support can be obtained.

The situation, unfortunately, is much worse if your friend is not a U.S. citizen.  In that case, if they have the ability to seek employment through the training option on an F-1 visa, then they may be able to pursue the same strategy.  Some professors, however, will attempt to effectively enslave foreign students through visa-related threats, and this can result in very difficult problems.