a friend of mines tire was slashed last night, and i was wondering if a 2.1 would fit on a rim that came with a 1.5. does anybody know for certain it would work?

depends on the rim but more than likely, it will fit. give it a test fit, it isnt that much trouble. make sure the tube you buy is suitable up to your tire size.

agree--should be fine. My commuter is a mid-'80s Bridgestone MB that was my main mountain bike for years. On the original rims, I've run tires from 2.3 to 1.25 without any problems.
FWIW, I buy only one size of 26-inch tubes, 26x1.5. Works with everything. They're rubber; they stretch.
One possible problem--you don't mention what kind of bike it is. Some hybrids might not have enough clearance between the fork blades or chain stays for a tire that big.

i didnt mention the brand being as i'm not sure of the brand.

I'm going to disagree with the other posters.

Knowing nothing about the bike involved except that it came with a 1.5 tire, I very much doubt a 2.1 tire will fit the bike frame.