The INS requires a financial proof of ability to pay for their tuition. This usually range about 1-2 years of tuition+room&board+expenditures, thats where the $50K totals to. A financial sponsorship statement usually accompany a financial proof of the availability of this fund. Its not written in any law that the funds must come from a family, although the INS is famous to interpret the law anyway they want to. I also never heard of the term limit on the funds, but this is might be something new that they implement, or simply local practice on the Shanghai/Guangzhou/Chengdu/etc consulate to increase chances of obtaining the F1 visa.
One of the acceptable financial proof usually includes Certificate of Deposit. What I will do is open a Certificate of Deposit with a maturity in 2004, with 40K comes from your GF's family and the other 10K coming from you. I will recommend the CD opened under you and your GF name, you and your GF's mom name. The CD and the financial statement should provide valid existence of financial funds for the purpose of obtaining the F1 visa. After your GF's cousin got the visa, you can revoked the CD and pay a small fine, which I'm sure your GF's family will be willing to pay (you're doing them a service afterall). You might not lost anything at all from your original funds, IIRC the penalty is taken from the interest already accumulated in the CD.
I would double check to ensure that the consulate really needs some proof of a fund that will be available until 2004. Judging from the term (1.5 - 2 years), is he coming for an Undergraduate or Graduate program ? If he's coming for Graduate program, he should have an assistantship lining up (if not, then he's simply a lazy SOB

) and there should not be a problem pulling the funds away after he got his visa).
One thing I would be worried about is the INS SEVIS system that handles security and monitoring on international student. If your GF's cousin doesn't pass that system, he can forget about entering the US in a student visa. The system check can also be very lengthy and could cause significant delay. If he's planning to come for the Spring 2003 semester, he might not make it in time.