Re-Read original post
Having gotten new info, I've edited this:
It's a JOINT computer (BOTH using it), and it's HIS money. She is, however, his long-time girlfriend/fiance (they're getting married).
This means:
1) HE has to use it, too. And if he WANTS a mac, why should her wanting a Dell take precidence? I'm not saying she shouldn't have any imput, but he's not buying it for her. He's buying it for THEM. If he was buying it for her and her alone, then let her have what she wants. If it's for both their use, then they MUST examine both, test out both, and not just blindly go for one or the other.
As for the "pick your battles" argument--that works if you're talking about where to go for dinner or a movie or buying a blanket for the bed. But for a computer? No. That's a lot of money and something they'll both use a lot. I would be furious if my significant other made me spend MY money on a computer that crashes, is vulnerable to viruses and impedes MY work and uses an OS I can't stand JUST because they're too scared to try something new. That is NOT an acceptable sacrifice for an acceptable reason.
2) As written in the original post, it would seem to be HIS money. But they're going to marry so I'm a bit fuzzy on this. If they both work and earn about equal money, then she should buy her own Dell. If he works and supports them, then they need to jointly decide. If they both work but he makes more money, again, they need to jointly decide.
I would still urge him to take her shopping with him, show her both Dell and Mac and how they work, show her how cute and slim and pretty the iBook is and that it can handle everything THEY want it to handle. He should NOT disregard her opinion in this joint decision, nor make her fear that he made a stupid mistake they both have to live with. In the end, however, I strongly feel that no good can come out of letting her, alone, make this decision. Not if he's really, really, really wants a Mac and will regret the money he spent if he doesn't get one.
I suspect that he's going to feel sick to his stomach every day he looks at that Dell if he buys one, regretting and kicking himself for not getting an iBook. I seriously doubt that she's going to feel the same if they buy an iBook instead of a Dell.