Well, you can inform your friend that his belief is a commonly cliched one used to rationalize both treating younger people like emotional china dolls as well as the perceived need for the AoC laws.

Young girls (and children and adolescents in general) change much faster than the typical adult does. This means that if the hypothetical relationship you speak of ended, it's much more likely that it did because the girl grew out of the relationship emotionally and moved on. Look at how AGs and, at times, LGs start relationships with peers, only for it to last a very short time as one or the other loses interest and moves on to the next exciting thing in their life. How often do you see them so devastated from these every day break-ups that they "can't handle it" and get "scarred for life"? Conversely, how many adults in comparison do we see handling break-ups well? Hence, contrary to popular belief, I highly doubt that it's the older partner who takes on the bulk of the emotional risk in a hypothetical intergenerational relationship, because younger girls tend to have strong feelings that are often nevertheless mercurial and fleeting in nature. As Harbor Boy always used to say here on the board (where has he been lately?), younger girls tend to live for the moment much more than they do with starting emotional connections of this nature that last extremely long term.

Given the above, it's always puzzled me to no end when people commonly assume that when a young girl falls in love with a much older man, it's likely to be for life, and that somehow a break up with a much older partner would have a far greater and more intense emotional impact on her than the break up of a relationship with a peer would. But these assumptions, however illogical or unfounded, are necessary to help rationalize the continuation of not only the AoC laws, but the taboo behind it in general.