It's probably worth trying some of the herbs on a conservative basis as long as clotting factor is available if needed. But they probably will not help at all. The problem is not the blood vessels. It is with the blood itself, it is missing key components of clotting factor VIII or IX or both. If the hemophilia is severe enough, the herbs probably will not have any impact at all. Strengthening the blood vessels to help with bruising may help a bit, but I really have no idea.
My husband never gave yarrow a good trial to see if it helps with bleeding. I guess it would be something that he would have take every day. He's found that the Oxymetazoline Hydrochloride in Nostrilla works really well in stopping nose bleeds. We got confirmation from his hemotologist that the Oxematazoline actually works as a clotting factor, it keeps the clot from breaking down too quickly. He will not sit forward and let the blood drain out of his nose. He lets it drain in his stomach and after all that blood, he won't even consider swallowing any herbal capsules and he's a big baby when it comes to taking tinctures.
Like I said in my previous post, the Oxymetazoline Hydrochloride in Nostrilla works so well for my husband, he's not had to go to the ER for humate in several years. He's only needed the clotting factor for surgery. The humate is given as an infusion pre-op and post-op. The humate works so well that the surgeons have commented on how little bleeding occurred during surgery. Nostrilla works really well for my daughter for her heavy periods and it allows her to take nsaids for cramps. I've tried all kinds of herbs (cramp bark, haw bark, kava, lobelia) and intestinal antispasmodics rx's (bentyl) that aren't nsaids and none of them really work as well as aleeve. She's just started taking a supplement with extra b6 (200mg) and hopefully that will releve all of her PMS symptoms and she won't need the aleeve any more.
Nostrilla isn't something that can be taken on a daily basis though so I don't think it would be appropriate for your friend's son.