Put it to you like is, if there isn't a strong mutual attraction before, one that wasn't acted upon out of respect for the homie, then don't bother. A close homies girl should be a friend and best, as both your takes on the relationship with that guy will constantly be on both your minds.
If I had to give a solid, solid timeframe? 8 months. She wouldn't be dating all the while, and within 8 months you should be able to speak to her without her bursting into tears. From there, take it slow.
Either way you do it, you might feel like a scumbag. She didn't leave him, he didn't leave her, he DIED. They didn't get a chance for the relationship to sour, so if things ended with a lot of good memories behind them, she is gonna love that dudes even in death, and you will be a constant reminder. Move on or wait 8 months"