Non issue in this situation. Actually bringing it up to counter this situation is rather childish. If you have a problem with a boarders horse - it needs to be communicated at the time. Not just when you feel you need something to put them in their place.
Trade her a set of polos. Done deal.
I agree mls.
Yep, I would never leave my horse in the cross ties to go and chat on my phone, and I wouldn't leave my hard earned horse tack/supplies laying around to allow the farm dogs to get into - but being as petty as this is, and childish as this is, I would just trade her a set of polo's and just move on.