Buy an AWP without armor, and you’re making a ‘glass cannon buy.’ It’s an enormous risk—you’re turning yourself into a slow-moving cannon with no protection, not only to damage, but to the accuracy-disrupting effects of aim punch. Avoid AWPing without full armor except in the most desperate straits, like when your opponent is on the verge of winning the match.
If your teammate in competitive mode has bought an AWP, reconsider buying another one. There are situations where the map will dictate otherwise (double AWPing as CT on de_dust2 is viable due to its long sight lines on A, B, and mid), but the economic risk is rarely worth the potential gain. In casual mode on the other hand, buying an AWP is merely a matter of buying cheaply for a round or two to save up for Big Green.