ETA: I am a little surprised that I am in the minority on this. I think that as parents, our role is to guide them in making their best choices, not let them learn the hard way - not at 15, anyway. In this case, it is not worth missing out on a whole season of a sport to satisfy a demanding girlfriend.
I would tell my son that he was going to the lacrosse tryouts, and that he was going to be late for the party. I would tell him that he could use mom as an excuse for why he had to go late to the party, if he needed to.
I would also tell him that he might want to reconsider a girlfriend who flipped out over something so trivial as being late to an *all day*! party, and who expected him to skip a whole season of lacrosse for her all day party.
At 15, you still have a reasonable amount of control over your son. No way would I allow my kid to miss the tryouts for a sport he loves to satisfy the whims of his demanding girlfriend.
You will probably have to be the bad guy on this, since hubby won't get involved. I strongly disagree with your hubby, and with everyone else who thinks you should stay out of it.
p.s. If the situation were reversed, and this was a 15 year old girl being forced to miss her tryouts for her demanding boyfriend, would we be okay with that? Certainly not. I'm all for life lessons -- they are fundamental to my parenting philosophies -- (and I so rarely disagree with my beloved Suz), but there are two ways to learn life lessons: by doing, and by not doing. At this age, the more productive way to learn this lesson, imo, is for the boy to play his season of lacrosse, and thinking all the while, 'I'm so glad I didn't miss out on this because of that party.' Then next time he's in a similar situation, he will have a better understanding of why he should not kowtow to the whims/demands of others.