My husband had a heart stent placed in his heart last friday? Is there a chance it could just block up?

Yes. Chance is low if one takes meds as prescribed. Should in all likelihood be taking clopidogrel, prasugrel, or another anti platelet agent. These are the kinds of questions you should discuss with your doctor before and after stent placement. Your husband may have had this discussion for you!
Yes. 1% of all stents clot off abruptly causing heart attack and risk of death. If a medicated stent is used, 5% of medicated stents will re narrow which typically presents as recurrent chest pain but not a heart attack. If the stent is not medicated, the renarrowing rate is 20% or more.