motdakasha that's excellent advice! In fact, it's so excellent, my gf and I were just discussing that earlier today. Great minds think alike!
We were discussing why they were actually together, and imho (and she concurs) HE, is comfortable being in the relationship (what's not to like... she's a quality gf, and one that is almost pampering him) and she, on the other hand, is a bit needy and vulnerable in general (confidence here is an issue, but as much as one tries to boost a girls confidence... it takes time?). Anyway, we were discussing widening their scope of friends. It's a wonder that you figured this out without all the facts! Allow me to elaborate.
One reason why Nicole is so vulnerable is because 3 months ago, fairly early in their relationship i suppose, they had a fight. Their relationship had not been going well, and i suppose they just got TOO much of each other. He was here too often in essence. Either way, it happened to be 'that-time-of-month' and she got a bit emotional and said something like "perhaps we should break up then". His response was 'okay'. THE BAD NEWS is that within 24 hours she was calling him up and wanting to get back together... Long story short, this caused a MAJOR falling out of her friends for complicated reasons. They lost a lot of respect for her, and (i thought them a bit over-critical) among many other reasons i suppose that had just been building up about her not paying enough attention to her friends. And then here she is chasing after some 'guy' that she has just begun dating. Anyway, when the smoke and dust settled, nobody was happy with her, including my gf. We took this in stride though, and after talking it out, we decided that we can't change what she has done, and we felt that we should at least support her when her other friends have left her.
A BIG reason i wanted to support her, is that it is too likely that if ALL her friends leave her, she will turn to HIM more and more and well... that's just not good. I gave him a good mano y mano talk and gave him the heads up. I made it clear to him that i have no opinion of him, but that i would look out for Nicole, and made it even more clear that i would not stand for a break-up and get-back-together relationship (which i feared might happen) and made it clear what i expected of him. Either way, Nicole lost 3 or so good friends, and already not being an outgoing girl, she really only has the three of us (Bob, me and my gf) left. Her other two pretty good friends are a good 3 hour drive away.
ANYWAY, thanks for the solid advice, we're putting off the letter for a little while (maybe a few days) see if we come up with anything. Perhaps we can get Nicole to... 'see the light'. If not, hey, more friends = good anyway.