Yes. I would totally break up love over a number. However in this case there are other reasons. A 21 year old woman who has been raped and molested and tells young teenage boys on the internet about it has real problems.
Your friend is 15, he was probably playing with micro machines two years ago. He has absolutely nothing to offer a 21 year old woman other than a sense of control over a relationship that she may not feel with a partner her own age. I know this because I used to date younger women (although not illegally so) and the exact reason I did was because younger women were less intimidating than ones my own age.
This young lady in question is emotionally damaged and pursuing an internet relationship with a 15 year old boy won't help her recover, it will only make it worse and can easily lead to her ending up in prison if even a single Skype session gets naughty and your friend's parents find out about it.
While what this relationship does to your friend is to give him an inappropriate sense of what is normal in a relationship. If the relationship does somehow last until he turns 18 then he will still end up losing out as he will move away from his home to be with a 24 year old woman who has severe emotional problems that he couldn't possibly hope to deal with as an 18 year old.
The real divide isn't truly over the number 18. That is just the common age of consent number that everyone jumps to. Most likely because most people reach that age about the time they graduate from high school.
The parents of most teenagers generally assume that they are going to college. In fact 65ish percent of them do go. Of course half of those end up dropping out eventually. However from my experience the older partner in these relationships in question is almost never a college student, they either work a menial job, or are unemployed basement dwellers. If they were college students then they wouldn't need to go sniffing around high schoolers 5 and 6 years younger than them since they would be surrounded with plenty of peers that could date instead.
Even when both partners truly love each other, no one is abusive and the relationship really lasts until both are "adults" the younger partner almost always ends up making major life sacrifices. They almost never make it through college and often end up looking to their older partner for the sort of guidance and assistance that a young adult should probably get from their parents instead of someone who makes bad choices.
I have a friend who ended up with a 15 year old girlfriend when he was 21. He was the classic older boyfriend type, being that he had a few community college credits, was no longer pursuing schooling and had a job with no future. He was (and still is) a smart guy but he just wasn't doing anything with his life and wasn't all that likely to. He wasn't trying to take advantage of her for sex and he truly did love her. The parents didn't really approve of him, but they tolerated him.
The relationship actually lasted. The girlfriend (who was pretty smart herself) was offered full scholarships to multiple schools, but of course she chose the one 2 miles from where the boyfriend lived instead of the half dozen better schools she could have attended. When she returned to college for her second year there was some mild mixup with room assignments and for some reason she wasn't assigned a room and the student who was working in the housing office didn't do anything about her complaint. So of course she turned to her older boyfriend for help and started staying with him while she tried to get her room situation resolved through the unhelpful student working in housing office. She gave up after about a week and just stayed with him all the time. Of course she didn't have a car and relied on him to get her to school. By the end of the school year she had failed all her classes and lost her scholarship. Eventually the boyfriend quit his job over a dispute with his boss and ending up having to move back into his parent's basement. His parents wouldn't let the girlfriend come too unless they were married so they got married then and there and moved into his basement.
Every single worry her parents had (other than pregnancy) came true. She threw away her scholarship, dropped out of school and ended up married at 20 years old. If she didn't have the older boyfriend then none of that would have happened. When the housing office messed up her room assignment she would have called her parents or spoken to a teacher and gotten the problem solved that day. Instead she went to her boyfriend whose "help" ended up being the catalyst of dropping out of a school that she was previously getting a 4.0 grade average at.
Update - 5 years later. Said couple I was talking about was living in a (US) state that was not native to them. The guy was a prison guard and she was tending bar. They decided to move back to their home state. They both finished out their work schedules, which allowed her to leave a few days early. That was two years ago, he never saw her again.