Sound advice. Not only that, at 18 he’s only just about learnt which bits go where! I think back to when I was that age and I was ...... at sex! Not worth ruining a friendship of a 20 second fumble that ends in bitter disappointment!
Cheap,option, buy her the dress, and nag her to wear it.
More expensive option, buy her the dress and a photo shoot, let the photographer show her how hot she Simon it, then take her to a really posh restaurant for dinner.