Obviously she would not be able to use the seat reservation, but if her 2nd class friend also foregoes a reservation and they can find a pair of unreserved seats in 2nd (a big if on some dates/times/routes), it would not be a problem to occupy them. The conductor will think it strange, but a voluntary downgrade is acceptable.
Next time you will book for both of them, right? Booking separately almost guarantees that the seats will not be together.