Any decision you make is entirely up to you, no one can tell you how to behave.
But please bear in mind that when you made the decision to to the LighterLife programme you decided to abstain from food for 14 weeks. I feel that the aniversary is one of many social challenges that you will be faced with during your 14 weeks.
LL is very much about abstaining from food so you can learn how to deal with situations in an alternative way. I'm sure that if you were to explain to your boyfriend that you need him to support you, he could find a way of celebrating that didn't involve food.
I was faced with a similar situation when it was my birthday so I understand how you feel, but once I'd made the decision to stick to the plan and got through the day I felt really very proud of myself. Life does not need to revolve round food and drink
The common problem with breaking abstinance once is that it's hard to get back on track and often people end up breaking it on a regular basis after that first time.
With all that said...I'm not here to tell you what to do....can only tell you what I learned and others will come and give you a different oppinion. As I said....the decision is yours, but I hope you have a lovely fun evening regardless and happy anniversary to you when it comes x