I am perfectly strait but I certainly know my way around the Thai bar scene. If you paid a bar fine to get the person out of the bar what you got is a hooker. It is not always customary to talk about money for services in Thailand. This does not mean money is not expected, it is just nicer for everyone if you don't talk about it. If your friend enjoyed his evening a nice tip descreetly left under his companions belonings while said companion was in the shower would be approprate under these circumstances. This is a polite way to handle a sex for money situation in a friendly way without everyone focousing on the money. Hard core types will engage in serious negotiations but nice type persons will leave payment as they say "up to you". I'd guess 1500 Baht would be about customary. Less may not offend the person if they had fun too. Up to you.