How to send a text via iMessage instead of regular text?

I have an iPhone 4s and I'm connected with my iTunes account and everything. My friend just upgraded from Android to iPhone 5s, so I changed her number in my contacts from 'mobile' to 'iPhone'. But now when I text her it still sends as SMS. I've already put in her iTunes email in my contacts as well. I've also tried turning iMessage on and off and resetting my phone. It doesn't have anything to do with the Wi-Fi because we're both connected and I can also text everyone else who has iPhones without Wi-Fi. I have no idea what the problem is. Any help or advice would be appreciated!

When you changed the number from mobile to iPhone that doesn't make it an iMessage. What you did there was simply change the label for that number.

The problem here is if she set up iMessage on her phone & if her number is registered as one of those numbers for iMessage.