This is the 6271st most frequent Russian word.
украинцу
The word "украинцу" is the dative singular form of "украинец," which means "Ukrainian" (a man from Ukraine).
Here, 'украинцу' functions in the dative case to indicate the beneficiary of the assistance.
Я видел, как вы помогли украинцу с переездом.
I saw how you helped the Ukrainian with the move.
In this context, 'украинцу' is again in the dative case, specifying to whom the information about the project was communicated.
Ты рассказал украинцу о нашем проекте.
You told the Ukrainian about our project.
Here, 'украинцу' is in the dative case, indicating the indirect object (to the Ukrainian) of the action of giving a book.