Should I text this girl and what should I say?

I know it's comfortable to text someone because you can think of what to say and how you would like that message to come across, but speaking from a girls perspective, there is no better way for a guy to communicate with you by talking (in person). I understand this is just early stages but I'd suggest you grow some confidence in what you think is alright to say to a "love-interest" instead of relying on us to write the message for you, because when it does come to speaking to her, you'll get a bit stuck. Don't be nervous or keep procrastinating about the fact that she's your friends ex, if he says it's alright, then it's alright, so act normal and be casual! Good luck!!

Yes, you should text her and your friend also gave you his blessings.