It depends on your mate, if he is still getting over her dont do it, if he doesnt give a toss then it might be ok, but maybe worth asking him subtly first? I've had two of my best friends now go off with my gf's whilst we were together because I was working away and its a really f-ing horrible thing for them to do.

Another one of my friends went off with another mates gf the day they broke up, that was bad and they didnt speak for a year (I still dont speak to the wankers who did it to me, and feel like decking them when i see them).

On the other hand, if they were dating for like 3 days, it doesnt really count, or if he dumped her cos she was a squico that is ok too, ive done that and found out quickly that she really was a nutter and got outta there myself pretty quickish!