A friend wants me to make cupcake bouquet centerpieces for her wedding, how will buttercream frosting hold up? For how long? 

Depends which buttercream you use.  If you use American buttercream it will hold up all day.

ABC can hold up even longer than a day.  Much longer.

ABC?

American Butter Cream

pls share recipe of butter cream for cup cake bouquet. ..once I made but butter cream got melt

Try Indydebi's recipe here on CC. It holds up GREAT.

http://www.cakecentral.com/recipe/6992/indydebis-crisco-based-buttercream-icing

Try the Swiss butter Cream recipe.  To be honest it is not just the texture but the taste as well.   You tube has got a brilliant recipe .  it is much easier than italian meringue butter cream and taste great.  Tip: Butter have to be room temp. and also cut it in blocks and add it block by block.   Good luck !