Can my friend date her long lost adopted brother?

It's an odd situation for her and we've talked about it a lot. Usually she comes to me for advice, but this is something I honestly don't know how to answer.

My friend was adopted when when she was 12. She got enrolled in our school and that's how I met her. That's how she also met a friend of ours that she was interested in. Over the years, we've all been a good group of friends. Now we're 21 and the two of them have decided to give their relationship a try since there's always been something there over the years. Well, before making everything official, they wanted their parents to meet up at a restaurant for a nice dinner. She brought her parents and he brought his mother.

Come to find out, her adopted dad used to date his mom back in the day and he wasn't ready to be a father, so he wasn't around when he was born. He went on with his own life and she gave birth and raised him on her own.

My my friends were very upset about all of this and demanded a DNA test. Well, the results are 100% he's his dad. (Honestly they even look kinda alike which is something I never noticed until they stood side by side.)

Both of my friends feel very weird about the situation and there's still some tension that will take awhile to settle.
Can my friend date her long lost adopted brother?
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