Usually most will have no issues of photographers having some food and/or a seat at one of the tables, and even personally invite you it. There are others who will ensure that their photographers are well fed before the event starts, as many knows that there will be little time for them to eat.
BUT, do note that are some minorities who are really calculative, and only expect you to work, not to eat. And also those who are status conscious, that dislike photographers sharing the buffet/ food with their 'distinguished' VIP guests. So it is best to know the host you're working for, first. On the other hand, there are also photographers who pigged on the food for the guests, in quite an unglamorous way. I've seen all this.
When I was first asked to do my relatives, friends and even office colleagues wedding shoots, eating is the last thing on my mind if i can help it. It is just the way I am when I put myself in that scenario and that's because I want to be the best at what I do. I want the best for me and them walking away from this event thus I tend to not sweat the small stuff.
That's not to say I will be take five sometime. ( I still need to go toilet heh )for me when I do the wedding or some event for companies I worked for ( no usually never for money but i do get the odd ang pows or added value to my job) I am more concern if I miss a crucial happening more then anything. For equipment I need to be sure they work or have a backup. It's no excuse to blame your gear when bad things happen. You agree to do the job then you have to do it. Be that gear or food.
So I tend to eat a big meal before I go shoot, I stash a water bottle on me or drink the odd cup at a buffet or even at a sit down where they did reserve a seat for me but i hardly sit to eat. I treat it as my buffet table for a quick bite or gulp. Some folks at the table does me a favor to put a small share of each course of food on my plate even. If I am not at a sit down wedding dinner but doing the ritual home-to-home take the bride, go temple or church eg.. I pack chocolate bars for quick energy and a few buns in my bag when I remember I feel hungry.
But then again that is the same for how you view work and life. To do your best, make no excuse and be prepare to know your stuff and your responsibility to deliver on that promise. Sure I am not paid so why put out 110% effort for friend sake? Each of us will have to find that answer for ourselves to live with. I know I have friends who asked me because they think I am good enough for what they need, at times yes because I am cheaper as they can't afford a pro and they just pay for my batteries,film and cost of processing, some can afford but they choose me still, some are my best bros or lady friends so they know I will not let them down for what i can do. For work, you do it because you feel you can contribute without being calculative, to help your career or just to get closer to your working colleagues. Most of all., because you love photography. Hope we have not forgotten that to begin with. heh.
But these days, the expectation is a lot more given what modern DSLRs can do, video options and the number of photo "standards" that comes with wedding packages, we now are seeing what use to be a solo photographer doing it all, it has expanded to at least a 2-3 or more operation to take on shooting all day on location, special couple shooting on locations and photo editing.
Given all that, friend or not, you have to be clear with what your friend's (or whoever) are in syn when ti comes to expectations of work. If not, things may not go well during the shooting event and worst, after the event when it is all said and done. Just be clear, even for no money, there will always be EXPECTATIONS. If you already have ready-made excuse to use in case you fail to deliver then you are not ready to take your photography to the next level.
Good example, look at the mess in the other thread about a wedding project that went thru a 3 party engagement which end up with you say, I say, they say.
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But that's just me.