3Bears how awful for you. I hope you are getting some RL support - that is lovely of your friends to cheer you up, you have good friends .
If you are feeling nervous about leaving DS2, why don't you change the "night out" to a nice meal out over some wine, rather than a big piss up? That way, you can still have a drink and be fairly normal on your return home, plus you'll only be out for a couple of hours returning around midnight, rather than at 3 in the morning. Just a thought - I'm sure your friends would change plans for you.
Secondly, regardless of what you are eating, your milk will not be rubbish. Without getting all technical or making you feel bad, but think of those in 3rd world countries who have a very poor diet and little water; they are still able to produce milk which is nutritious. They continue to feed their babies despite eating very little and their milk has the right amount of vitamins and goodness that the baby needs.
A few weeks of eating a bit crap or not eating well, will definitely not make your milk rubbish BUT you should be looking after yourself. Sometimes it is the notion of having to make food - go and get yourself some nice ready meals, fresh pasta which is quick to cook and some jar sauces. You must try and eat something (and porridge is good for supply and a good easy food to make. Add a nice dollop of syrup and it will make you feel much better I'm sure)
As you probably know, giving the odd bottle of formula (if he eventually takes a bottle) will, in the end, affect your supply, should you do this on a regular basis. This is something to consider if you did want to continue BF'ing. However, the odd bottle for a night out, shouldn't have too much affect - just remember to pump when you get home!