You make the girl so pleased to be with you that when she gets those texts she doesnt give a crap. If you arent together, you cant do anything. That guy is disrespecting you, he isnt your friend. Get over him, dont be jealous. You are with this girl not him.
Do not crowd this girl to make her attention on you, in fact do the opposite. Give her space. If you bring this up with him, he might go to her and use the 'X said that he thought I was coming on to you, he got really upset' which makes you look aweful, and might even make her wonder 'I thought you were coming on to me...' and her interest level might raise toward him.
You need to be a man, men dont worry. Just don't get attached to this girl, if she goes off with him you can't stop it. Just laugh at them if they do and be that guy that she can't get out of her brain when she is off with this other guy, because you DIDNT CARE. You might get her back after this, but why would you want a girl that will run off at the next sign of someone else being interested.
If she does something, NEXT
If she doesnt, great.
But by being that jealous guy, you hurt yourself and relationship. By worrying, you do nothing but cause damage. This is fact. If she is the type of girl that would want to run off with someone else would you even want her? No. So There is NO POINT worrying. You would be better off giving her a bit of space and freeing your mind from her a bit because you sound like your getting attached. Iv been pretty much in your situation.
However, if she then asks to be exclusive you have to tell her you dont want to be exclusive and commit until she can respect that you don't like her texting this guy because of X,Y,Z.
PM me if you want help