How bad is it to have a gigantic bruise on your inner arm?

My friend went to donate plasma the other day and they messed up and went all the way through his vein. He passed out and bled a lot. It’s not a week later and he has a HUGE bruise stemming from thw inside of his elbow. It’s about ten inches long total and maybe five inches wide. It was black and is now fading slightly. He hasn’t gone to get it checked out medically because he doesn’t think it’s a big deal. Is it?