I don't have a problem with this, in principle. As long as the intentions are true and noble, and he is not using her (or indeed, her using him) for sex/status etc, then why not. At 15, someone is probably emotionaly mature enough to take responsibility for their body and heart. They may not make a huge success of it, but they are old enough for it to be their business and no-one elses. If she was dating a 25 year old, or older, one might have a legitimate issue with the principles of it. But can anyone rationalise why this is wrong? Men mature (emotionaly) later than women, so he might be just the right age for her in that respect. If she was any younger, then her ability to take responsibility, use her judgement and be held accountable for her actions may be called into question, as it is very easy to be carried away by one's illusion of love, especialy when one does not have much experience.
When I was 15, I was with a 17 year old. When I was 17, I was with a 24 year old, when I was 18, I was with a 44 year old. Was I used? Abused? Taken advantage of? No. Sure, you do get bad eggs, but really, it is not very healthy to go through life looking for problems, scared to explore.