Bleeding Skin

This is just too weird, and I need feedback asap so I can make the decision to take or not take this 16 year old boy to the Dr. I'm not his mom, he's my son's best friend. I've taken him before, and his parents went nuts. They NEVER think he's really sick, as a result, he had mono for 4 months, and now is considered chronic. My new concern is that his skin bleeds. One day, he and his girlfriend were horsing around, and she sucked on his back, then screamed as she wiped the blood from her mouth...yes, HIS blood. At first she thought it must be hers, but rinsed her mouth, and it wasn't hers. She wiped his back, and there was no mark whatso ever. So of course she did it again on his shoulder, and sure enough, after a few seconds, blood ran down his shoulder. Being totally freaked, they came and showed me, this time on his forarm. Sure enough, blood, but no marks. He is a robust, seemingly healthy boy with plenty of muscle mass and average weight. He told his parents, and they said it was no big deal!! My concern is that the mono, which went untreated for so long, left some kind of other condition or permanent condition. As anyone ever heard of this? If it could be serious, I'll risk the wrath of his parents and possible legal action and take him to our local Med Center. I did it before, and I'll do it again if necessary. Thanks for taking the time to read this rather lengthy post.