I did tell his mother to march him off to a STD clinic for testing, Round. 
If you have unprotected sex without vaccination then sorry, but you are at risk of contracting all sorts of stds including HPV, which causes cervical and mouth and throat cancers.
We do not know what the 14 year old boy involved has been up to or with whom. We can probably assume he experiments quite a bit though, right? With lots of other partners, right? Since 'experimenting' is so common apparently, and it seems not to matter with whom...
According to you, the 13 year old shouldn't be 'forced to be honest'
about having sex with someone else while at the same time enjoying the status of a 13 year old boy who has a girlfriend? God forbid that a boy should face consequences of his choices.
The girlfriend should be given the information she needs in order to make a solid decision about whether to continue to get vaccinated if she is not already, or whether to continue to be this boy's beard.
Incidentally, how do you think she will feel when the truth about her boyfriend emerges?
Do you think she will feel used and humiliated?
Does that matter at all to you?
Do you think it's kinda cute that he is cheating on this girlfriend, or that he is likely to expose her to HPV that may cause cancer years later, probably bang in the middle of her childbearing years when she has babies or small children to think about?
At what age do you think we should expect integrity from our children?
Evidence of respect for other people?
43?
62?
85?
Or maybe a little younger?
This boy only confided in his mother when he was caught red handed. The ship of mutual trust has well and truly sailed. We should not assume he and the girlfriend have not engaged in sexual activity of some sort.
'Experimenting' is rife, after all, isn't it?