Wow Milkshakes... Wow... Your story almost perfectly resembles the situation I'm in right now.... o.o
The girl (my absolute best friend, an INFP) I've fallen in love with the moment I laid eyes on her, about a year and three months ago (and still am in love with) also proceeded to have a relationship with an ESTP guy (who she met on that same night) for almost a year.
I'm still in love with her and only told her after their relationship had ended (not wanting to be the guy waiting for a relationship to break up before taking the girl...). On the other hand, I díd tell her I love her, though the feeling isn't mutual. We're still best friends, though she sometimes feels like she has lost her best friend and got me how I am now in return, a guy that just says yes to everything and accepts everything she does (which I don't). This might have something to do with the fact that we also kissed a few times. Though I get the feeling she doesn't think that anymore, so that's good
We're all members of the same student society, so we see each other almost daily.
The ESTP guy is a very quiet guy, very gentle. He also knows I'm in love with his ex, though he did not threaten or intimidate me, so that's a relief. XD He's also a very good friend of mine, which makes it quite awkward at times.
The INFP girl is a very kind girl, likes to work hard for the student society (often to the point of over-exerting herself, at which point either me or her ESTP ex jumps in to take care of her so she doesn't neglect herself). She also likes to get a lot of attention (which I, of course, am more than willing to give... XD)
Many people at the student society judge her because they think she's stringing me along, playing me for her own good. They are wrong though, as I've been in a relationship where, in hindsight, I probably was used as a sort of emotional outlet for my (probably ENFP) ex, so I know when I'm being used. (Also, I've already said no to the INFP girl a few times when I just did not have the time to help.)
And then there's me, the ESFP, a drama-queen, always wanting to be in the centre of attention (and not always getting it in a good way...). Almost everybody at the student society knows I'm in love with her, since I can hardly keep it hidden. People at the society generally like me, I have yet to find someone I can't get along with to a lesser or greater degree. I've been told I'm a good person who always has the best in mind for everyone.
One of my friends, also an INFP, came to me a few weeks ago while I was bartending (ESFP's make perfect bartenders, I am told), called me up for a short 1-on-1 and he warned me about the INFP girl and me and how we spend time together, as he had the idea that the INFP girl was using me, because I always help her out with stuff she asks me to do. I told him I can take care of myself well enough and that he should not worry about me (it felt good that somebody actually took the time to talk to me about it though, made me feel there are more people that care about me than I think
)
Now, anyway, after that wall of text... On to your question about how to approach and advise your ESFP friend.
I think the best thing to do is, as said before, ask him to have a chat with you somewhere private, so nobody else can hear or interfere. Tell him about your concerns (be sure to tell him it's YOUR feelings and idea, not what you think he is feeling or thinking).
Start sentences with "I feel ..." or "I think ..." and be sure to give your friend room to react as well. Just talk with him without raising your voice or he will do the same and that is something you do not want, as he'll probably not pay attention to what you are saying anymore, but how you are saying it, which will most likely get him riled up.
If this happens, it's best to tell him talking won't work like this and try again another day. Be sure he knows YOU are the one concerned about him and that YOU tell him because you want to help him. I, at least, am very sensitive to how friends think I'm doing, so if they think I'm doing bad, I'll try and work on it.
Also, make sure he understands your concern and, again, let him voice his opinion and feelings and listen to them.
I hope this helps you in some way