請問老貓 and other friends

My girl friend (F1)is graduating this summer and she got a job offer.The problem is that her OPT starts from July.Her "future" boss sort of pushes her to sign the labor contract(is "labor" correct?)right now.Can she do it right now,but mark something like "the contract will be valid from July to~~~~", or she has to wait until then to do so?Another issue is that her boss promises to support her with the process of H visa and green card.However,she has to work there for 3 years.This content is also writen in the contract.I am wondering:when something regarding H visa and green card writen in the contract,does that really mean something serious?Is it really protected by law as well as other contents?Coz I often hear that people are strongly advised to look for the job where the boss is willing to support the H or green card.I am not sure if the willingnesss should be verbal or in the paper?