Are there any bughouse chess engines? A friend told me its very hard to implement a bughouse chess engine because it requires much more processor power than a regular chess engine.

Are there any bughouse engines out there ? if not, what are the reasons ?
There are certainly several chess engines, but as a result of the additional complexities referenced by @TonyEnnis, these engines are weaker than reasonably strong players. It's extremely difficult to take into account the different scenarios that can arise from two positions as pieces switch from board to board every move. The resulting search tree increases at an exponential rate even greater than that of standard chess.

The engine that I'm most familiar with is Sjeng which claims that it has an ELO of around 2000 on FICS, equivalent to a strong human player.