Doc tends to be on the chivalrous side, and yes, will accept a modicum of 'mistreatment' from the ladies, but in this case, he's also taking it at face value. This girl states she's a long-time friend of his new girlfriend, and professes to care enough about her to help keep her from making more bad decisions.
Doc understands he needs to get on this girl's good side- if she has one - to not cause any trouble between... well, the three of them, really.
Also, there's a bit of cognitive whiplash- Doc arrived thinking he was possibly going to have to "rescue" (possibly literally) Cara from a nasty, poorly-run, badly-managed store, only to find that said girlfriend apparently actually owns the place.
Then he walks in expecting to see Cara, only to be greeted by an empty store, and confronted by this little lady who already seems to be in a bad mood toward him.
Yes, if the situation involved going to a potentially rival paintball store, and being confronted by the guys working there, sure, creative malletery and potential small explosions might have been involved.
But in this case, he's gotta 'suck it up' a bit, and sit down to answer a few questions.