If she feels like she's 'missing out on things' because she's with her bf, then perhaps he's not the one for her. I think she needs to talk to her bf and explain that things are moving too fast for her - although, saying that, if they stay together, then surely she'll still be 'missing out'?? tbh, I don't really get the whole missing out on things bit that people so often talk about - if you love someone enough to want to be with them for the rest of your life, what on earth are you missing out on? Surely there's nothing better than this? Well - that's my opinion anyway. Back to the point.... it's a difficult situation to be in, and certainly one that's very difficult to approach. however, if you are uncomfortable with her using you as an alibi, I think this is a perfect opportunity to have a real heart-to-heart with her about it. This may be difficult, and it she may resent it for a while, but try to make it clear that you have her best interests at heart, and that you want what's best for her, but stress that she needs to talk to her bf about this, and if she won't, then she at least has to stop using you as an alibi.
People might think that that's what's being a friend is all about - covering for your friend if she's in trouble, but I don't think that's the case in this situation. Friends deserve to know the truth, and it's her best friends that will tell her straight up. I'm not saying be accusational about it, but try to make her realise that what she's doing will ultimately end up hurting both her and her bf, and does she realise that by catching up on what she's missing out on, she runs a high risk of losing something that she already has. What's more important to her?
Try and be diplomatic, be kind, but get the point across. She may say, it's her business, her life, but at least you've set her thinking about it, and she'll probably thank you in the end.
That would be my advise - as I say, it's a difficult position to be in, but I hope what I've said helps should you choose to act on it. x