I had this problem with XP. It really annoyed me.
I mean I rarely take the DC's to pubs, when we were still togetehr around once a week we would go to the local and I'd take the DC's and meet my friends and their dc's for lunch so he seemed to think it was okay for him to continue taking them on his access days.
I told him I wasn't having it, firstly he was having them once a fortnight, for that time he was meant to be spending quality time with them and I did not want him consuming alcohol as he was in sole charge of them. Secondly, I didn't want them around drunk people!
When I had taken them before we had sat in the restuarant part with all my friends and their children and none of us drank so it was a completely different scenario, but to him he'd just though 'VS took them to the pub, I will'.
So I had to explain the difference in him taking them.
He no longer has them though, he didn;t stop taking them to the pub, he also used to cancel access with half hours notice even though we would drive 40 miles to take them to him etc.
Now he is only allowed to see them when they go to his mum or sisters.
I'd tell your friend to try talking to him at first, he may just not have thought about how it's different for the NRP to spend their one day of access in a pub in comparison to the resident parent spending a few hours in one.
I'd tell her to suggest that if he doesn't stop taking DS to the pub then he will only be allowed supervised access.
I doubt she'll need witness statements that eh ahs been in the pub either, though if she is worried, then turning up at said pub a few hours after DS has been dropped off may be a good idea. I know when I went into the local once to see if the DC's were there his friends were only too happy to tell me where he was, and not in a bad way, they just thought I needed to see him and the kids so were being helpful.