At the worst, drunk clients are unpredictable and potentially dangerous. He should have been turned away, or in this case, you should have left shortly after arrival.
At the least, drunk clients are incapable of performing sexually, leading to frustration and possible anger on their part. At best, they will overextend their welcome trying to get off. At worst, they will insist upon bb services because they just "know" that they will be able to cum then.
If you choose to stay with a drunk client, for whatever reasons, do not hesitate to leave when things do go sideways. Don't hesitate to keep the full donation either. It is still your time, you were prepared to stay and do your best, he blew it. If he seems drunk but only maudlin rather than dangerous and you decide to stay, just be a friendly ear, offer a massage, and a hj rather than attempting to do fs. It most likely won't work anyway, insist on silence because no doubt he will want to talk on and on about how "if only you did...." it would work for sure. If the hj doesn't work then so be it. He has had a nice massage and a friendly conversation.
The fact is, drinking before, or during appts should NEVER be encouraged. Don't make a habit of bringing a bottle of wine, or accepting a glass of anything. Tell them you don't drink if you have to. It seems like a nice gesture, but honestly flowers are a better gift. If you think you need a drink to get in the mood, then the sp is doing something wrong lol.
GHB doesn't care how carefully you screen your clients, they are strangers and should be treated with caution first and foremost. Some people like to use poppers, e, or ghb for personal use. Some clients have actually put ghb or similar on the condom or themselves before the sp goes in for a bj. Never underestimate how careful you can be with just a little common sense.
There is no need to reinvent the wheel, here. All these topics have been discussed by both sides in many ways at many different times. When you decide to go into this industry full force, in would serve you well to study what those who have gone before you have to say lol.