Springcleanish "Presumably the forty quid was drinks for all three of them."
Doesn't sound like it to me - from the OP "cost her about £40. (This was separate from the cost of her own drinks )"
I do, however, quite like this idea "when will you be able to pay me,"
I'm not sure that asking about a night out "on you" is such a good idea, as the chances are the friend would either (a) worry about OP's daughter drinking her under the table or (b) it would just go wrong some other way (blazing argument, who knows)...
Also, if someone is getting more than half-way smashed (20 quid on booze sounds plenty to me) then the odds are it may be awkward to keep track (!)
OK, am out of touch as to silly prices in clubs and some pubs, but also can think of far better things to spend 20, 40, whatever quid on.... The last 25 quid I spent on booze was for 2 dozen bottles of Stella cidre, and a bottle of Irish cream, a fraction the price of Baileys, and the more appealing because of the lower cost {something like 5 instead of 15 quid} :)