It's hard to tell someone you don't trust human nature and hormones without also telling them you don't trust them too.

If she starts seeing this guy "As a friend" there's a very strong probability that she'll become emotionally invested in him after a fashion.  That string is all it takes to build a foundation that becomes a problem where she finds herself with urges she doesn't currently have and may end up cheating on you with him even though she doesn't see herself doing it right now.

Relationships are a pathway and evolve over time.  By setting herself up in a position for her relationship with him to evolve unchecked, she's setting herself up for some serious headaches.

She's already creating problems by texting him constantly.  A bond -is- forming, and it -is- something to be concerned about.  Something you should discuss openly before it becomes a seething jealousy issue.

See also: Get a small novelty ruler, so 6" looks like 12" and hold it up to your dick.  Use her phone and send him a snapshot.  Follow it up with "Oh, s\*\*t dude, sorry.  I meant to send that to my wife.  She's got my phone"