Don't be an idiot and have some guy random text her. You haven't thought this through. She will either think it is a scam or she will figure out you put someone onto her and you will end up in a argument.
First thing you should do is make sure you are doing everything possible to fuck here like a man should fuck his wife. Go out and read online how to have sex and make her orgasm. buy Vibrators dildoes and use your fingers and lick her clit.
I guarantee you that her slutty side will emerge the more you do that. Then watch boring Chick Flicks that have torrid affairs and move to porn. Only then you can start planting the seed of other guys and you start talking about it. The texting in my opinion is a horrible idea because you pushing her to have secrets. If you want her to fuck other guys you should be able to openly talk about this.