I don't know all hadiths, but I am pretty sure you won't find anything about this specific case. The person was not supposed to touch his aunt ' s/uncle private parts in the first place. It is like asking if you can eat a lamb rib after eating a pork rib and repenting, this case is only covered by the first interdiction.
Note that the responsibility of the person depends a lot of his / her age. It is possible that the uncle / aunt in question, due to his / her age, is more responsible.
Here is why the person should not marry his / her cousin :
Consanguinity is bad for the health of the children. The oumma is big enough, the person can pick someone else.
It will be very hard for the cousin to forgive this once found out
It will be even harder for the cousin to forgive to his / her father / mother. Put yourself in the cousin 's shoes, the situation is just as bad.
If you know someone in this situation, tell him / her not to do it, I cannot picture this wedding going well.