Personally, I think women riding mens boards is different than men riding womens boards. Maybe I'm off-base here but that's my gut feeling.
And a girl weighing 125 lbs is no different than a guy weighing 125 lbs. It's just that I'm assuming he will probably grow a lot more and quicker than a girl would. But if the board is only going to be used for about a season while he progresses, it would be fine. A kids board might work but don't they only go up to like 140 cm? that seems pretty small.